Physiologia Epicuro-Gassendo-Charltoniana, or, A fabrick of science natural, upon the hypothesis of atoms founded by Epicurus repaired [by] Petrus Gassendus ; augmented [by] Walter Charleton ...

SECT. III.

PERSPICUITY and OPACITY we well know to be Qualities not praecisely conformable to the Laws of Rarity and Density;* 1.1 yet, insomuch as it is for the most part found true (caeteris paribus) that every Concretion is so much more Perspicuous, by how much the more Rare; and è contra, so much the more Opace, by how much more Dense; and that the Reason of Perspicuity can hardly be understood, but by assuming certain small Vacuities in the Body interposed betwixt the object and the eye, such as may give free passage to the visible Species; nor that of Opacity, but by conceding a certain Corpulency to the space or thing therein inter∣posed, such as may terminate the sight: therefore cannot this place be judged incompetent, to the Consideration of their severall originals.

By a Perspicuum [〈 in non-Latin alphabet 〉〈 in non-Latin alphabet 〉] we suppose, that every man un∣derstands that Body,* 1.2 or Space, which though interposed betwixt the Eye and a Lucid, or Colorate Object, doth nevertheless not hinder the Transition of the Visible species from it to the Eye: and by an Opacum; that which obstructing the passage of the Visible Species, terminates the sight in it self.

We suppose also, that (according to our praecedent Theory) the Spe∣cies Visible consist of certain Corporeal Rayes emitted from the Object,* 1.3 in direct lines toward the Eye; and that where the Medium, or inter∣jacent space is free, those Rayes are delated through it without impediment; but, where the space is praepossessed by any solid or Impervious substance, they are repercussed from it toward their Original, the Object. And hence we inferr, that because the total Freedom of their Transmission depends only upon the total Inanity of the Space intermediate; and so the more or less of freedome trajective depends upon the more or less of Inanity in the Space intermediate: therefore must every Concretion be so much more Perspicuous, by how much the more, and more ample Inane Spaces it hath intercepted among its Component particles; which permit the Rayes freely to continue on their progress home to the Eye.

This we affirm not Universally, but under the due limitation of a Caeteris Paribus, as we have formerly hinted. Because, notwith∣standing a piece of Lawn is more or less Perspicuous, according as the Contexture of its Threads is more or less Rare; and the Aer in like man∣ner is more or less pellucid, according as it is perfused with more or fewer Vapours: yet do we not want Bodies, as Paper, Sponges, &c. Which though more then meanly Rare, are nevertheless Indiaphanous; and on the

contrary, we see many Bodies, sufficiently Dense, as Horn, Muscovy-glass, common glass, &c. which are yet considerably Diaphanous.

Now, that you may clearly comprehend the Cause of this Difference, be pleased to hold your right hand before your eye, with your fingers somewhat distant each from other; and then looking at some object, you may behold it through the chinks or intervals of your fingers:* 1.4 this done, put your left hand also over your right, so as the fingers of it may be in the same position with the former; and then may you perceive the ob∣ject, at least as many parts of it as before. But, if you dispose the fingers of your left hand so as to fill up the spaces or intervals betwixt those of your right; the object shall be wholly eclipsed. Thus also, if you look at an object through a Lawn, or Hair Sieve, and then put another Sieve over that, so as the holes or pores of the second be parallel to those of the first; you may as plainly discern it through both as one: but, if the twists of the second sieve be objected to the pores of the first, then shall you per∣ceive no part of the object, at least so much the fewer parts, by how much greater a number of pores in the first are confronted by threads in the se∣cond. And hence you cannot but acknowledge that the Liberty of in∣spection doth depend immediately and necessarily upon the Inanity of the pores; the Impediment of it upon the Bodies that hinder the trajection of the Rayes emitted from the Object: and yet that to Diaphanity is required a certain orderly and alternate Position of the Pores and Bodies, or Particles. This considered, it is manifest, that the Reason why Glass, though much more Dense, is yet much more perspicuous than Paper, is only this; that the Contexture of the small filaments, composing the substance of Paper, is so confused, as that the Pores that are open on one side or superfice thereof, are not continued through to the other, but variously intercepted with cross-running filaments: as is more sensible in the Co••texture of a Spunge, whose holes are not continued quite thorow, but determined at half way, (some more, some less) so that frequently the bottome of one hole is the cover of another, as the Cells in a Hony-comb: but, Glass, in regard of the uniform and regular Contexture of its particles, which are ranged as it were in di∣stinct ranks and files, with pores or intervals orderly and directly remaining betwixt them; hath its pores not so soon determined by particles opposite∣ly disposed, but continued to a greater depth in its substance.

Though this make the whole matter sufficiently intelligible, yet may it receive a degree more of illustration,* 1.5 if we admit the same Conditions to be in the substance of Glass, that are in a Mist, or Cloud; through which we may behold and object, so long as the small passages or intervals be∣twixt the particles of the Vapours, through which the rayes of the visi∣ble species may be trajected, remain unobstructed: but yet perceive the same so much the more obscurely, by how much the more remote it is; because, in that case, more impervious particles are variously opposed to those small thorow-fares, that obstruct them, and so impede the progress of most of the rayes. For, thus also Glass, if thin, doth hinder the sight of an object very little, or nothing at all; but if very thick, it wholly termi∣nates the progress of the species: and, by how much the thicker it is, by so much the more it obscures the object. And this, only because Glass, con∣sisting of small solid Particles, or Granules, and insensible Pores alternately situate, hath many of its pores running on in direct lines through its sub∣stance

to some certain distance; but sometimes these; sometimes those are obturated by small solid particles succedent, when at such a determinate Crassitude, it becomes wholly opace.

* 1.6And this gives us an opportunity to refute that vulgar Error, That the substance of Glass is totally Diaphanous, or that all and every Ray of the the Visive Species is trajected through it, without impediment. To demon∣strate the contrary, therefore, we advise you to hold a piece of the finest and thinnest Venice Glass against the Sun, with two sheets of white paper, one betwixt the Sun and the Glass, the other betwixt the Glass and your Eye: for, then shall all the Trajected Rayes be received on the paper on this side of the glass, and the Reflected ones be received on that beyond it. Now, insomuch as that paper, which is betwixt your eye and the glass, doth receive the Trajected rayes, with a certain apparence of many small shadows intercepted among them; and that paper beyond the glass, doth receive the Reflected rayes with an apparence of many small lights: therefore we de∣mand (1) from whence can that species of small shadows arise, if not from the Defect of those rayes, that are not transmitted through the Glass, but averted from it? (2) Whence comes it, that in neither paper the Bright∣ness or Splendour is so great, as when no Glass is interposed betwixt them; if not from hence, that the reflected rayes are wanting to the nearest, the tra∣jected ones to the farthest? (3) Whence comes it that some rayes are refle∣cted, others trajected; if not from hence, that as a Lawn sieve transmits those rayes, which fall into its pores, and repercusseth others that fall upon its threads: so doth Glass permit those rayes to pass through, that fall into its pores; and reverberate those, that strike upon its solid particles? And what we here say of Glass, holds true also (in proportion) of Aer, Water, Horn, Vernish, Muscovy-glass, and all other Diaphanous Bodies.

Notes

• * 1.1

  Art. 1. The opportunity of the present speculation, concerning the Causes of Perspicuity and Opacity.
• * 1.2

  Art. 2. The true No∣tions of a Per∣spicuum and Opa••um.
• * 1.3

  Art. 3. That every Concretion is so much the more Diapha¦••ous, by how much the more & more ample Inane Spaces are in∣tercepted a∣mong its par∣ticles; caeteris pa••ibus.
• * 1.4

  Art. 4. Why Glass, though much more Dense, is yet much more Diaphanous, than Paper.
• * 1.5

  Art. 5. Why the Dia∣phanity of Glass is gradually diminished, ac∣cording to the various de∣grees of its Cra••••••tude.
• * 1.6

  Art. 6. An Apodicti∣cal Confutati∣on of that po∣pular Error, that Glass is to∣tally, or in every particle, Dia∣phanous.