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\.4 New Thetiryabout Light aniColours ; hy Mr,Is. Newton,n.8o,f. 3075.

B, 83. f. 4061Tig. ;o.

CHAP. III.

0 P T I C K S.

I. TN the Year 1666 (at which time I applyed my felf to the Grinding of. Optick Glafles of other figures than Spherical,.) Iprocured me a Triangu-lär Glaß Prism, to try thercwith the Celebrated Phaenomena öf Colours. Andin order thereto, having darkncd my Chamber, and made a (mall Hole inmy window-fhuts, to Jet in a convenient quantity of the Sun’s Light, I placedmy Prism at it’s entrancc, that it might be thereby Refracted to the oppositeWall. was at first a very plealing Divertisement, to view the Vivid and In-tense Colours produced thereby j but aster a while applying my (elf toconfiderthem more circumspectly, I became sorprised, to see them in an OblongForm ; which, according to the received Laws of Refractions, I expected (houldhave been Circular. They were terminated at the fides *with streight Lines,but at the Ends, the decayof Light was (b gradual, that it was difficilst to de-determine justly, what was their Figure j yetthey seemed Semicircular.

Comparing the Length of this Colour’d Spcilrum with its Breadth, I foundit aboutfive times greater, a Disproportion sb extravagant, that it excited me toa more than ordinary Curiosity of examining, from whence it might procced.

I could scarce thinlc, that the various thickness of the Glass, or the Terminationwith Shadow or Darkness, could have any influencc on Light to produce such anEffect ; yetl thought it not amiss, sirit to examine thosc Circumstances, and (btry’d, what would happen by transinitting Light through parts of the Glass ofdivers Thicknefles, or through Holes in the Window of divers Bignesses, orby setting the Prism without, sb that the Light might paß through it, and beRefracted, before it was terminated by the Hole : But I found none of thosc■circumstances material. The falhion of the Colours was in ali these Cases theferne.

Then I suspected, whether by any Unevenneß in the Glass, or other contin-gent Irregularity, these Colours might be thus dilated. And to try this, I tookanother Prism like the former, and sb placed it, that the Light pasting thro’them both might be Refracted contrary ways, and sb by the latter returnedinto that course from which the former had diverted it. For by this means Ithought, the Regulär Effects of the first Prism would be destroyed by the se-cond Prism, but the Irregulär ones more augmented, by the Multiplicity of Re-fractions. The event was, that the Light, which by the first Prism was difiu-fed into on Oblong Form, was by the second reduced into an Orbicular one, withas much Regularity as when it did not at all paß through them.

. That this Experiment may be better apprehended,let E G, design the Window ;

F, the Hole in it, through which the Light arrives at the Prisms; ABC, the firstPrism, which Reffacts the Light towards P T, painting there the Colour in an Ob-

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