fill
h
Of Light and Colours.
ht passing through it, it follows, thatthe several Sorts of Rays will have
their
3*S
Platexxxvi.Fig- 9*
'°ur - the next Size less will be still more refracted,
9-Ud appear Yellow at Y, and thus the Refraction willProceed in the Green at G, the Blue at B, the Indiga?t I, and the Violet-coloured Rays at V ; which as they-'re most refracted, are thereby proved to be the leastPf all in Magnitude.
6 . I shall now proceed to shew, since the Sun’sFight is variously refrangible, what the particular De-Htec of Refraction is which every Species of Rays un-P^rgoes, and the Sines of those Angles respectively. In° r der to this it must be considered, that the Sine of In-Fdence is the fame in all ; and that when the Inci-dence is such as that the Ray F K, upon the first Re-faction, shall pass in the Direction parallel to the upperSide of the Prism G H, the Refractions made at each?>de of the Prism are equal, and equal to the refracting^tigle of the Prism GDH; all which is evident fromhhat was demonstrated in Annot. CXVI.
. 7 . Also it is known by Experience, that when the1 rifin A B C is held with its Axis perpendicular to thejUn-Beam, and then turned round upon its Axis, the■rtiage or coloured Speftrum will first descend to a cer-■Wn Limit, where it will become stationary, and thena( cend to the fame Place as at first; whence it appears^ a in, that since the Altitude of the Image above the**ace where the Sun-Beam would fall, were the Prism^ay, is owing to the Sum of the Refractions made at~ ac h Side of the Prism, while the Image descends thellr n of these Refractions decrease, and when the Image^ Ce nds the said Sum must increase,j. 8 . Consequently, since the Image falls twice upon thePlace in one Rotation of the Prism, there are two,°sitions of the Prism wherein the Sum of the Refrac-'°ns at its Sides are equal; and these happen when the pig. ^/• ^gles of the incident Beam HDL and O D L arei Uc si as will cause the refracted Parts D G and D F to,e e sluaily infclined to the Sides of the Prism, but con-
trary