Color, according to the physicists, is a property of light, by which it produces, depending on the various configurations and speed of its particles, vibrations in the optic nerve, which being propagated into the sensorium , affect the mind [ âme ] with different sensations. [1] See Light.

Color can also be defined as a sensation of the soul [ âme ] excited by the application of light to the retina, and differing according to the degree of refrangibility of the light and the speed or magnitude of its parts. See Sensation.

The properties of light will be found in the article Light.

The word color , properly speaking, can be envisaged in four different ways: either insofar as it designates a particular disposition and affection of light, in other words of the corpuscles that constitute it; or insofar as it designates a particular disposition of physical bodies to affect us with this kind of color or that; or insofar as it designates the excitation [ ébranlement ] produced in the organ by these or those luminous corpuscles; or insofar as it indicates the particular sensation which is the sequel of that excitation.

It is in this last sense that the word color is ordinarily taken, and it is quite evident that the word color taken in this sense designates no property of bodies, but only a modification of our mind; that whiteness, for example, redness, etc. , exist only in us, and not at all in the bodies with which we nevertheless associate them by a habit contracted in our childhood. It is a very singular thing, and worthy of the attention of metaphysicians, this penchant we have to associate with a material and divisible substance what really belongs to a spiritual and simple substance; and nothing is perhaps more extraordinary in the operations of our mind than to see it go outside itself and extend, so to speak, its sensations onto a substance to which they cannot belong. However that may be, in this article we shall hardly envisage the word color insofar as it designates a sensation in our mind. All we can say on that matter depends on the laws of the union of mind and body, which are unknown to us. We shall say only two words on a question that several philosophers have proposed, which is whether all men see the same object as the same color . It would appear so, yet one will never demonstrate that what I call red is not green for someone else. It is, moreover, rather plausible that the same object does not appear to all men as an equally vivid color , as it is rather plausible that the same object does not appear equally large to all men. That is because our organs, though differing little from each other, nevertheless have a certain degree of difference in their strength, their sensibility, etc . But that is enough on this matter; let us come to color insofar as it is a property of light and of the bodies that reflect it.

There are great differences of opinion about colors between the Ancients and the Moderns, and even between the different schools of philosophers today. Following the opinion of Aristotle, which was the one formerly followed, color was regarded as a quality residing in colored bodies, and independent of light. See Quality.

The Cartesians were not satisfied with this definition: they said that since the colored body was not immediately applied to the organ of sight to produce the sensation of color , and that no body could affect our senses but by an immediate contact, therefore colored bodies could contribute to the sensation of color only by means of some medium, which being put into motion by their action, transmitted that action to the organ of sight.

They add that, since bodies do not affect the organ of sight in the darkness, the sentiment of color must be occasioned by light that puts the organ in motion, and that colored bodies must be considered only as bodies that reflect light with certain modifications, the difference of the colors coming from the different texture of the parts of the body, which makes them able to give this or that modification to light. But it is above all to Mr. Newton that we owe the true theory of colors , one which is founded on solid experiments, and which offers the explanation of all the phenomena. Here is what that theory consists in.

Experience shows that rays of light are composed of particles of differing masses; at least some of these parts, as we could hardly doubt, are much faster than others, for when a ray of light is received in a dark room F E (Optics plate [1] , fig. 5) on a refracting surface A D , that ray is not wholly refracted to L , but splits and spreads, as it were, into several other rays, some of which are refracted in L , and others between L and G , such that the slowest particles are the ones which the refracting surface diverts most easily from their rectilinear course towards L , and the rest, depending on their greater speed, are turned less, and pass closer to G . See Refrangibility.

Furthermore, the rays of light that differ the most from each other in refrangibility are also those that differ the most in color : this is a truth recognized by an infinite number of experiments. The most refracted particles, for example, are those that constitute the violet rays, and this, according to all appearances, because those particles, being less rapid, are also those that excite the retina the least, excite in it the fewest vibrations, and affect us consequently with the least powerful and least vivid sensation of color , as is violet. Contrariwise, the particles that are least refracted constitute the rays of the color red, because those particles, being faster, strike the retina more forcefully, excite the most sensible vibrations, and affect us with the most vivid sensation of color , such as is the color red. See Red.

The other particles, being separated in the same manner, and acting according to their respective speeds, will produce by the different vibrations they excite the different sensations of the intermediate colors , just as the particles of air excite the different sensations of the sounds according to their different respective vibrations. See Vibrations.

We must add to this that not only do the colors most distinct from each other, such as red, yellow, blue, owe their origin to the different refrangibility of the rays, but that the same applies to the different degrees and shades of the same color , such as those that are intermediate between yellow and green, between red and yellow, etc.

Moreover, the colors of the rays thus separated cannot be regarded as simple adventitious modifications of those rays, but as properties that are necessarily associated with them, and which consist, according to all appearances, in the speed and size of their parts; they must therefore be immutable and inseparable from those rays: in other words, those colors cannot be changed by any refraction or reflection.

That is what experiment confirms in a palpable manner; for whatever effort has been made to separate by additional refractions any colored ray at all emitted by the prism, no success has been obtained. It is true that apparent decompositions of colors are sometimes made, but that is only the colors that we have formed by combining rays of different colors ; and it is not surprising then that refraction makes us rediscover the rays we had used to form that color .

Hence it follows that all the transmutations of colors which one produces by mixing colors of different kinds are not real, but simple appearances, or deceptions of sight, since as soon as the rays of these colors are separated, we have the same colors as before. That is why blue powders and yellow powders mixed together seem to simple sight to make green; and without giving them any alteration, we easily distinguish, with the aid of a microscope, between the blue and yellow particles.

We can thus say that there are two sorts of colors : the ones, primitive, original, and simple , produced by homogeneous light or by rays that have the same degree of refrangibility, and are composed of parts of the same speed and mass, such as red, orange, yellow, green, blue, indigo, violet, and their gradations; the others secondary or heterogeneous , composed of the primary ones or a mixture of rays of different refrangibility.

We can produce secondary colors by means of composition, just like the primitive colors as for the tone or shade of the color , but not with respect to permanence or immutability. In this manner we form green with blue and yellow, orange with red and yellow, yellow with orange and yellowish green; and in general, with two colors that are not far apart in the series of colors generated by the prism, we rather easily succeed in making the intermediary colors . It is important to know too that the more composite a color is, the less it is vivid and perfect; and that by compounding it more and more a point is reached where it disappears entirely.

By means of compounding we can also succeed in forming colors that resemble none of those of homogeneous light. But the most singular effect that the compounding of primitive colors can yield is to produce whiteness. It is formed by using in a certain degree rays of all the primitive colors ; that is why the ordinary color of light is white, because it is nothing other than the assemblage of light of all the colors mixed and melded together. See Whiteness.

Refraction given by a single refractory surface produces the separation of light into rays of different colors ; but this separation becomes much more considerable and strikes in a completely sensorial way when double refraction caused by the two surfaces of a prism or any piece of glass is used, provided these two surfaces are not parallel. As the experiments we do with the prism are the touchstone of the whole theory of colors , here is a summary of them.

1. The rays of the sun transmitted through a triangular prism exhibit on the opposite wall an image of different colors , of which the principal ones are red, yellow, green, blue, and violet. The reason for this is that the differently colored rays are separated from one another by refraction; for the blues, for example, marked (Optics plate [1] fig. 6 ) with a dotted line, after separating from the rest in d d by a first refraction occasioned to the side ca of the prism a b c (or by the first surface of the globe of water a b c, fig. 7 ), are even further separated into e e by refraction in the same direction, produced by the other side of the prism (or the second surface of the globe a b c ). It happens on the contrary in the glass plane a b c f, figure 9 (or on the prism g l o, fig. 8 placed in another situation), that the same blue rays that had begun to separate by the first surface in d d become, by a second refraction, parallel to their first direction, and consequently remix again with the other rays.

2. The colored image is not round but oblong, its length being five times its breadth, when the angle of the prism is about 60° or 65°. The reason for this is that this image is composed of all the particular images exhibited by each different kind of rays, and which are placed some above others, according to the force of reflectivity of these rays.

3. The rays which exhibit yellow are turned further from their rectilinear course than those that exhibit red; those that exhibit green more than those that exhibit yellow, and so forth up to those that exhibit violet. Because of this principle, if we turn on its axis the prism on which the sun’s rays are falling, in such a way that the red, yellow, etc., fall successively on another fixed prism placed at a certain distance from the first, at twelve feet for example, and the rays of these different colors have previously passed one after the other through an aperture placed between the two prisms, the broken rays which these different rays will yield will not all be refracted to the same spot, but some above the others.

This simple and nonetheless decisive experiment is the one by which Mr. Newton was led out of all the difficulties into which the first ones had thrown him, and entirely convinced him of the correspondence that exists between the color and the refrangibility of the rays of light.

4. The colors of the rays separated by the prism cannot change naturally nor be destroyed, although these rays pass through an illuminated medium, cross each other, and happen to border on a deep shadow, whether they are reflected or broken up in any manner; whence we see that colors are not modifications arising from refraction or reflection, but immutable properties that belong to the nature of the rays.

5. If by means of a lenticular glass or a concave mirror you manage to bring together all the different colored rays that the prism exhibits, you form whiteness; yet these same rays which all gathered together have formed white yield, after being recombined, in other words beyond the point where they cross, the same colors as they yielded when exiting the prism, but in reverse order, because of the crossing of the rays. The reason is clear: for the ray, being white before being separated by means of the prism, must again be white by rejoining the parts which refraction had separated from each other, and this reassembly can in no manner tend to destroy or alter the nature of the rays.

Likewise, if you mix in a certain proportion some red color with some yellow, green, blue, and violet, you will form a composite color which will be whitish (that is, about like the color you get by mixing white and black) and which would be entirely white if some rays were not being lost and absorbed. You again form a color almost white by staining a circle of paper with different colors , and making it rotate briskly enough that you cannot distinguish any color in particular.

6. If you cause the sun’s rays to fall obliquely on the inner surface of a prism, the violet rays will be reflected and the red ones will be transmitted, which comes from the fact that the rays which have the greatest refractivity are those that are most easily reflected.

7. If you fill two hollow prisms, one with a blue fluid, the other with a red fluid, and join them together, they will become opaque, although each of them taken separately is transparent: because one of them transmitting only red rays, and the other only blue rays, they must transmit none at all when they are joined together.

8. All natural bodies, but principally those that are white, being viewed through a prism, appear bordered on one side with red and yellow, and on the other with blue and violet; for these outlines are nothing other than the extremes of so many images of the whole object as there are different colors in light, and which do not all fall in the same place because of the different refractivity of the rays.

9. If two prisms are placed so that the red of one and the violet of the other fall onto the same paper, the image will appear pale; but if you view it through a third prism, keeping your eye at a proper distance, it will appear double, one red and the other violet. Likewise, if you mix two powders, one of which is perfectly red and the other perfectly blue, and cover a body of little extension with this mixture, the body viewed through a prism will exhibit a double image, one red and the other blue.

10. When the rays that pass through a convex lens strike a paper before they meet at the focus, the edges of the light will appear reddish; but if it is struck after they meet, the edges will appear blue: for the red rays, being the least refracted, must join the farthest away, and consequently be closest to the edge, when you place the paper before the focus; whereas after the focus, it is on the contrary the blue rays joined first that must then contain the others, and be toward the edges.

The colored image of the sun, which Newton calls the solar spectrum , at first sight offers only five colors : violet, blue, green, yellow, and red; but by shrinking the image, to make the colors more decisive and distinct, you see very well the seven: red, orange, yellow, green, blue, indigo, and violet. M. de Buffon ( Mémoires de l’académie, 1743 ) even claims to have seen eighteen or twenty;  [2] nevertheless there are only seven primitive colors , for the reason that in dividing the spectrum, following Newton’s proportion, into seven spaces, the seven colors are inalterable by the prism; and because dividing it into more than seven, the neighboring colors are of the same nature.

The proportional extension of these seven intervals of colors corresponds fairly well with the proportional extension of the seven tones of music. This is a singular phenomenon; but it would be wrong to conclude that there is any analogy between the sensations of colors and those of tones, for our sensations have no similarity to the objects that cause them. See Sensation, Tone, Ocular harpsichord, etc .

M. de Buffon, in the paper we have just cited, counts three ways by which nature produces colors : refraction, inflection, and reflection.  [3] See those words. See also Diffraction.

Colors of thin plates . The phenomenon of the separation of the rays of different colors exhibited by the refraction of the prism and other bodies of a certain thickness can further be observed by means of plates or thin lamellæ, transparent like the bubbles that rise on the surface of soapy water; for all those little lamellæ with a certain degree of thickness transmit rays of all colors , without reflecting any of them; but as they increase in thickness, they begin to reflect first the blue rays, and successively afterward the very pure green, yellow, and red ones. By further increases in thickness, they furnish in addition blue, green, yellow, and red rays, but a little more intermixed, and finally they come to reflect all these rays so well intermixed that they make white.

But it must be noted that in whatever place the reflection of a color occurs in a thin lamella, such as blue, for example, in the same place will occur a transmission of the opposite color , which will be in this case red or yellow.

We find by experiment that the difference of color which a plate exhibits does not depend on the surrounding environment, but only on the vividness of that color . Other things being equal, the color will be more vivid if the densest environment is surrounded by the rarest.

A plate, other things being equal, will reflect that much more light as it is thinner, up to a certain point, beyond which it will no longer reflect any light.

In the plates whose thicknesses increase according to the progression of the natural numbers 1, 2, 3, 4, 5, 6, 7, etc . , if the first, in other words the thinnest, reflects a ray of homogeneous light, the second will transmit it, the third reflect it again, and so forth; so that the odd numbered plates, 1, 3, 5, 7, etc . , will reflect the same rays, which their corresponding even-numbered plates 2, 4, 6, 8, etc ., will transmit. Hence one homogeneous color exhibited by a plate is said to be of the first order if the plate reflects all the rays of that color . In a plate three times thinner, the color is said to be of the second order . In another thickness five times thinner, the color will be of the third order , etc.

A color of the first order is the most vivid of all, and successively the vividness of the color increases with the color ’s order. The more the thickness of the plate is increased, the more colors and different orders are reflected. In some plates the color will vary with the position of the eye; in others it will be permanent.

This theory about the color of thin plates is what Mr. Newton calls, in his Opticks the theory of fits of easy reflection and easy transmission , [4] and it must be admitted that, ingenious as it is, it does not have anywhere near enough to convince and satisfy the mind entirely. Here we must be content with simple facts, and wait to learn or seek the causes until we are better informed about the nature of light and bodies, which means waiting for a very long time, and perhaps forever. However that may be, here are a few of the experiments resulting from the facts that serve as basis for that theory.

Colored rings of glasses. If you place two glasses of very large radius one atop the other, the air found between these two glasses forms a sort of thin disk which is not of the same thickness throughout. Now at the point of contact the thickness is zero, and you see black at this spot; then you see all around it several rings differently colored, and separated from each other by a white ring. Here is the order of the colors of these rings, beginning with the black spot in the center:

There are still other rings, but they fade progressively away.

By looking at the glasses from below, you will see colors in the places where the rings seem to be separated, and these colors will be in another order. See Musschenbroeck, Essai de Physique, §1134 and ff. [5]

Thus are explained the changing colors one observes in soap bubbles, varying as the thickness of such bubbles is more or less great.

Colors of natural bodies . Bodies appear to be of one color or another only insofar as they reflect only the rays of that color , or as they reflect more rays of that color than of others; or rather they appear to be of the color that results from the mixture of the rays they reflect. See Body.

All natural bodies are composed of small, transparent lamellæ, and when these small lamellæ are disposed with regard to each other in such a way that there is neither refraction nor reflection in their interstices, the bodies will be transparent; but if the intervals between these lamellæ are filled with a heterogeneous matter with respect to that of the lamellæ themselves, or if there are many refractions and reflections within the body, that body will then be opaque. See Transparency and Opacity.

The rays that are not reflected by an opaque body penetrate it, and there suffer innumerable reflections and refractions, until they at length unite themselves with the particles of the body itself.

Hence it follows that opaque bodies grow hot less as they reflect more light; whence we see that white bodies, which are those that reflect the most rays, become much less hot than the black bodies which reflect almost none at all. See Heat, Blackness, etc .

To determine the constitution of the surface of bodies, on which their color depends, it must be considered that the corpuscles or first parts of which these surfaces are made up are very thin and transparent; furthermore, that they are separated by a medium that differs from them in density. We can therefore regard the surface of each colored body as an infinite number of small, thin plates, in the situation of those of which we have just spoken, and to which can be applied everything we have said in that context.

Whence it follows that the color of a body depends on the density and thickness of the particles of that body contained between its pores; that the color is all the more vivid and more homogeneous as those parts are thinner; and that, everything else being equal, those parts must be thickest in red bodies, and thinnest in violet ones; that ordinarily the particles of the bodies are denser than those of the medium that fills their interstices; but that in peacocks’ tails, in some silk fabrics, and in all bodies whose color depends on the situation of the eye, the density of the parts is less than that of the medium; and that in general the color of a body is less vivid as it is rarer in relation to the medium contained in its pores.

Further, the different opaque bodies of which the lamellæ are the thinnest, are those which appear black, and the white bodies are those which are made up of the thickest lamellæ, or of lamellæ that differ considerably in thickness, and are consequently able to reflect all colors . The bodies made of lamellæ of median thickness between these first ones will be either blue, or green, or yellow, or red, depending on which of those colors they reflect in greatest quantity, absorbing the rest or transmitting them.

It is because of this last circumstance of reflecting or transmitting the rays of one color or another, that certain liquids, such for example as that of the infusion of lignum nephriticum, appear red or yellow by the reflection of light, and that they appear blue when they are placed between the eye and the light. It is the same with gold leaves, which are yellow in the first case and blue in the second.

We can further add that the change in color that occurs in some powders employed by painters when they are ground very fine, is no doubt owing to the perceptible diminution of the parts of these bodies produced by the grinding, just as the change in color of the lamellæ is produced by that of their thickness.

Finally, that singular phenomenon of the mixture of liquids from which different colors result could be owing to no other cause than the different actions of the saline corpuscles of one liquid on the corpuscles that constitute the color of another liquid; if these corpuscles unite, their masses will be thereby shrunken or swollen, and their density consequently will be altered; if they ferment, the size of the particles will diminish, and consequently the colored liquids will become transparent; if they coagulate, an opaque liquid will result from two transparent colors .

We see again easily by the same principles why, when a colored liquid is poured into a conical glass placed between the eye and the light, different colors appear in the different parts of the glass being viewed: for depending on whether the section of the glass is furthest from the bottom or point, there will be more rays intercepted; and in the top of the glass, in other words at the base of the cone, all the rays will be intercepted, and none will be seen except by reflection.

Mr. Newton asserts that one can deduce the thickness of the component parts of natural bodies from the color of those bodies, for the particles of the bodies must exhibit the same colors as lamellæ of the same thickness, provided the density is also the same. This whole theory is conjectural.

As for the particular properties of each color , see Black, White, Blue, etc.; see also Rainbow.

Colors that result from the mixture of different liquids or the arrangement of different bodies . When you infuse red roses for a short time with distilled alcohol, and pour over this still white infusion some acidic spirits of salt, like spirit of vitriol, oil of sulphur, spirit of sea-salt or of niter, or aqua fortis, but in such a small quantity that even the acid cannot be noticed, the white infusion will at once turn to a lovely rose-colored red. If you add to this red tincture some dissolved alkaline salt, as for example potash lye, or spirit of ammoniac salt, it will change to a lovely green; but it you add to the infusion of roses some vitriol dissolved in water, it will at once produce a tincture black as ink. Musschenbroek, essai de physique. [6]

If you infuse some gallnuts in water for a short time, such that this infusion remains white, and you add some common vitriol, or vitriol which has been burned in a flame until it has turned white, or has been reduced to red colcothar, [7] you will have at once a black tincture. If you add to this tincture a few drops of oil of vitriol or aqua fortis, all the black color will disappear, and the tincture will regain its original clarity. But if you add to this black liquid some drops of potash lye, this whole mixture will at once turn dark black, and to make it lose this blackness it will suffice to add a little acid spirit to it.

If you place on a dark blue paper a piece of white paper that has previously been lightly rubbed with aqua fortis, the blue will turn russet, and then pale. The same thing happens when you have written on blue paper with alkaline phosphorus. If you clarify some common violet syrup with water, and pour it into two different glasses, the syrup with which you will mix an acid liquid will become red, and the one to which you add an alkaline or salt will become green. If then you mix these two syrups thus changed, you will have a blue syrup, assuming you have used as much acid as alkali. But if the alkali dominates, the whole mixture will be green; and if the acid is in greater quantity, the mixture will become red. When you pour a little tarter salt solution on some sublimated mercury dissolved in water, that mixture becomes a thick, opaque red; but if you add to this mixture a little alkaline spirit or ammoniac salt, it becomes white once more.

If you also dissolve a little blue vitriol in a large quantity of water, such that the whole remains white and transparent, and next you add to this liquid a little ammoniac salt, you will see, once this mixture is made, the appearance of an elegant blue color ; but if you add a little aqua fortis, the blue color will immediately disappear, and the water will become clear and white. Finally, if you again add some more ammoniac salt, the blue color will reappear. When you pour an infusion of bou tea [ thé-bou ]  [8] onto some gold dissolved in ethered spirit of wine, a purplish form of chalk forms that precipitates to the bottom. When you dissolve some tin in aqua regia, and after clarifying this solution with water you add some drops of gold dissolved in aqua regia, you see appear a fine purple color most agreeable to the eyes. Those who wish to see a larger number of experiments on changing colors should consult Boerhaave’s chemistry; [9] others can also be found in the work of the philosophers of Florence; [10] finally one could do worse than to consult again on this matter the Philosophical Transactions , no. 238, § vi ; [11] Musschenbroek, ibid.

The infusion of gallnuts poured onto the solution of vitriol produces a mixture whose parts absorb all the light they receive without reflecting but a very small amount or none at all, whence it happens that this tincture appears black; but we do not know what is the arrangement of these parts. When you add to this tincture a few drops of aqua fortis, it becomes once again as clear as water, and the black color disappears, because the aqua fortis at once violently attracts the vitriol which separates the gallnuts, which then float in their water as they did before, leaving it all its clarity and transparency. As soon as you then add to this mixture some drops of potash lye, which being an alkaline salt acts strongly on the acid, they immediately attract the acid parts of the aqua fortis, which itself separates out from the vitriol it had attracted to it, so that the vitriol again thereby recovers the means of uniting with the parts of the gallnuts and produces the same black color as before.

The parts of the surface of a blue-violet paper have a determinate thickness and size, but the minute aqua fortis makes them thinner, or they separate some from the other parts, they must disperse the light rays that have a color different from the first ones, which makes the color blue change into a ruddyish color ; and as the particles of the paper become thinner by the day, and are as it were eaten away by the humidity of the air which adds to the parts of the aqua fortis, they must continually break up other colored rays, and consequently make the paper appear of another color . See Musschenbroek, Essai de Physique, p. 556 ff, from which this is taken. [12]

Accidental colors are colors that never appear except when the eye is forced, or has been too violently shaken. This is what M. de Buffon, in a very curious paper printed among those of the Academy of Sciences for 1743, [13] has called these sorts of colors , to distinguish them from the natural colors that depend solely on the properties of light, and are permanent, as least for so long as the exterior parts of the object remain the same. No one, says M. de Buffon, had made any observations about this kind of colors before Mr. Jurin, [14] yet they relate to the natural colors in several particulars; and here is a series of rather singular facts which he lays out for us on this subject.

1. When you look long and hard at a red spot or figure, such as a small red square on a white background, you see a sort of dim crown of green form around the red figure; and if you move your eye to some other part of the white background, ceasing to look at the red figure, you very distinctly see a light green but somewhat bluish square.

2. By looking long and hard at a yellow spot on a white background, you see a pale blue crown form about the spot; and moving your eye to another place on the white background, you distinctly see a blue spot the same size and shape as the yellow spot.

3. By looking long and hard at a green spot on a white background, you see a white, lightly purplish crown form about the green spot; and moving your eye to another place, you see a blue spot of pale purple.

4. By looking in the same way at a blue spot on a white background, you see a whitish, slightly reddish crown form about the blue spot; and moving your eye to another place you see a pale red spot.

5. By looking attentively in the same way at a black spot on a white background, you see a bright white crown form around the black spot; and moving your eye to another place, you see the figure of the spot exactly depicted, and of a much brighter white than that of the background.

6. By looking long and hard at a bright red square on a white background, you first see the small, light green crown already mentioned form; then, by continuing to look hard at the red square, you see the middle of the square discolor, and the sides become colored and form something like a red frame, much stronger and much darker than the middle; next, by moving back a little and still continuing to look hard, you see the dark red frame divide into two on the four sides, and form a cross [ croix ] of equally dark red; the red square then appears like a window traversed in its middle by a big cross shape [ croisée ] and four white panels, for the frame of this sort of window is of as strong a red as the cross shape. Continuing still to look intently, this appearance again changes, and everything reduces to a rectangle of a red so dark, so strong, and so vivid that it entirely offends the eyes; this rectangle is of the same height as the square, but not a sixth of its width. This point is the ultimate degree of fatigue that the eye can tolerate; and when you finally turn you eye away from this object, and move it to another place on the white background, you see instead of the real red square the image of the imaginary red rectangle exactly drawn, and of a bright green color . This impression subsists for a very long time, losing color only slowly, and remains in the eye even after it is closed. What we have just said about the red square also occurs when you look at a square that is yellow or black, or any other color : you see in the same way the yellow or black square, the cross and the rectangle; and the impression which remains is a blue rectangle if you have looked at yellow, a bright white rectangle if you have looked at a black square, etc.

7. No one is unaware that after looking at the sun, you sometimes transpose the image of that star for a very long time onto all objects. These colored images of the sun are of the same kind as those we have just described.

8. The shadows of bodies which by their essence ought to be black, since they are only the privation of light, are always colored at sunrise and sunset. Here are the observations which M. de Buffon says he has made on that subject. We shall cite his own words:

“In the month of July 1743, as I was occupied by my accidental colors , and wanted to see the sun, the light of which is more easily borne by the eye at its setting than any other time of day, to learn next the colors and changes of color caused by that impression, I observed that the shadows of the trees falling on a white wall were green. I was in an elevated spot, and the sun was setting in a mountain gorge, such that it seemed to me very low beneath the horizon. The sky was serene except for the setting sun, which though free of clouds was covered with a transparent curtain of reddish-yellow vapors. The sun itself was quite red, and its apparent size at least quadruple what it is at noon. Then I saw very distinctly the shadows of the trees that were twenty or thirty feet from the white wall, colored with a soft green with a little blue; the shadow of a trellis that was three feet from the wall was perfectly depicted on that wall, as if it were newly painted in verdigris. This appearance lasted nearly five minutes, after which the color faded with the sunlight, and disappeared entirely only with the shadows. The next day at sunrise I went to look at other shadows on another white wall, but instead of finding them green as I expected, I found them blue, or rather the color of the most vivid indigo. The sky was serene, and there was only a small curtain of yellowish vapors in the east; the sun was rising over a hill, such that it appeared to me risen above my horizon; the blue shadows only lasted three minutes, after which they appeared to me black. The same day I again saw the green shadows at sunset, as I had seen them the day before. Six days then went by without my being able to observe the shadows at sunset, because the sun was always covered with clouds; the seventh day I saw the sun as it was setting: the shadows were no longer green, but a fine azure blue. I observed that the vapors were not very abundant, and that the sun, having advanced for seven days, was setting behind a boulder that made it disappear before it could sink below my horizon. Since that time I have very often observed the shadows, either at sunrise or at sunset, and have seen them only blue, sometimes a quite vivid blue, at other times a pale blue or a dark blue: but constantly blue, and every day blue.” [15]

1. This article being largely a translation (with notable adaptations) by d’Alembert of the article Colour in Chambers, Cyclopaedia (vol. I, 1741), the translator has borrowed much of the English terminology from said article. That article uses the words sensorium and mind (which d’Alembert translates as âme ), as here. But âme also means soul , as in the next paragraph.

2. Georges Louis Leclerc, comte de Buffon (1707–1788), “Sur les couleurs accidentelles, » in Histoire de l’Académie royale des sciences (1743), Physique et histoire naturelle , pp. 1–8 ; and “Dissertation sur les couleurs accidentelles,” in Mémoires , pp. 147–58.

3. D’Alembert indeed takes the musical comparison in the previous paragraph from Buffon’s paper ( Mémoires , p. 149).

4. Isaac Newton, Opticks, or a theory of the reflexions, refractions, inflexions and colours of light (London: Smith and Walford, 1704), p. 81.

5. Pieter van Musschenbroek (1692–1761), Elementa Physicæ (1726); the French title was Essai de physique (1739; the link is to a 1751 edition). In the English translation by John Colson, The Elements of Natural Philosophy (London: J. Nourse, 1744), the reference here is to vol. I, pp. 199–200.

6. As d’Alembert indicates, these first two experiments and some others are taken from Musschenbroek (see note 5), §943; in the English edition, vol. II, pp. 95–96.

7. Colcothar : “The precipitate which remains after the distillation of oil of vitriol” ( Dictionnaire de Trévoux , 1752 supplement).

8. Thé-bou , “a kind of tea purchased in Nanking. The Dutch bring it to Europe, where it is highly esteemed” ( Dictionnaire de Trévoux , 1752 supplement).

9. Herman Boerhaave (1668-1738), Elementa chemiae (Leiden, 1732). There was an unauthorized French edition in Paris in 1724 under the title Institutiones et experimenta chemiae ; an English translation by Peter Shaw appeared in London in 1727.

10. See Marco Fontani, Mary Virginia Orna, and Mariagrazia Costa, Chemistry and Chemists in Florence: from the last of the Medici family to the European Magnetic Resonance Center (Springer, 2016).

11. Robert Southwell, “Several experiments about giving variety of tinctures to water, etc,” Philosophical Transactions , vol. 20, issue 238, 31 Dec. 1698, art. vi, pp. 87–90).

12. In the French version Essai de physique, 1739, vol. II.

13. See note 2.

14. James Jurin (1684–1750), had written an addendum to Robert Smith’s A Compleat System of Opticks (1738) entitled “On Distinct and Indistinct Vision.”

15. This passage comes from the Academy paper already cited in note 2, pp. 157-58.