Experiments and considerations touching colours first occasionally written, among some other essays to a friend, and now suffer'd to come abroad as the beginning of an experimental history of colours / by the Honourable Robert Boyle ...

EXPERIMENT IV.

The Triangular Prismatical Glass being the Instrument upon whose Effects we may the most Commodiously speculate the Na∣ture of Emphatical Colours, (and perhaps that of Others too;) we thought it might be usefull to observe the several Reflections and Refractions which the Incident Beams of Light suffer in Rebounding from it, and Passing through it. And this we thought might be Best done, not (as is usual,) in an ordinary Inlightn'd Room, where (by rea∣son of the Difficulty of doing otherwise) ev'n the Curious have left Particulars Unheeded, which may in a convenient place be easily taken notice of; but in a Darken'd Room, where by placing the Glass in a convenient Posture, the Various Reflections and Re∣fractions may be Distinctly observ'd; and where it may appear what Beams are Un∣ting'd,

and which they are, that upon the Bodyes that terminate them, do Paint either the Primary or Secondary Iris. In pursu∣ance of this we did in the above mention'd Darken'd Room, make observation of no less than four Reflections, and three Re∣fractions that were afforded us by the same Prism, and thought that notwithstanding what was taught us by the Rules of Catop∣tricks and Dioptricks, it would not be amiss to find also, by hiding sometimes one part of the Prism, and sometimes another, and observing where the Light or Colour Va∣nish'd thereupon, by which Reflection and by which Refraction each of the several places whereon the Light rebounding from, or passing through, the Prism appear'd ei∣ther Sincere or Tincted, was produc'd. But because it vvould be Tedious and not so In∣telligible to deliver this in Words, I have thought fit to Referr You to the Annexed Scheme where the Newly menrion'd par∣ticulars may be at one View taken No∣tice of.