28 On Vision. 



produced. Let a c (Plate I. fig. 9.) represent an aperture in the 

 window-shutter of a darkened room, through which «o, a beam 

 of solar light, is transmitted ; and def^ a prism placed behind 

 the aperture ac. Now the ray flc is not attracted, but repelled 

 at the lower edge of the hole, towards the higher angle of the 

 pnsm (/ ; in like manner, the ray na is refracted from the upper 

 edge of the aperture towards e, the refringent at)gle of the prism, 

 and becomes incident at r". It is evident that these two rays 

 are most refracted, and have the greatest polarity of all the rays 

 tliat compose t!ie beam of light ; and therefore, as the angle of 

 refraction is in each nearly equal, both rsys will be red. The in- 

 termediate rays, incident upon d e, will give a succession of co- 

 lour according to their refraction or polarity ; the ray next to r' 

 will be blue i', that next to b' will be yellow y . The same order 

 will follow with relation to the red ray at the refringent angle ; 

 the rav b" , that falls next to /', is blue; and that next to b'' is y", 

 or yeilow. All these rays, r' b' y\ y" b" r'', pass through thq 

 planes of the prism with a refraction corresponding to the angles 

 of their incidence ; and, agreeably to a well-known law of optics, 

 the angle of their emergence, at the opposite surface, ce, of the 

 prism, is respectively as the angle of their incidence. Hence, on 

 examining, the spectrum r' is incident at V, b' at B, and y' at Y ; 

 the ray ?•" falling with somewhat more obliquity upon the plane 

 de than the ray r', a portion of it is reflected in the directional, 

 — while the other passes through the opposite plane, and is inci- 

 dent at R. Now it is evident that the red which the spectrum 

 here exhibits, is produced by the ray ar" that comes from the 

 upper edge of the aperture ; the orange, which appears next 

 the red, at O, is a composition of the rays r" and y' ; the yellmu, 

 which is exhibited at Y, is produced by the mixture of the rays 

 y' and y" ; the green, at G, is composed of the rays b' and y"; 

 the blue, which is exhibited at B, is produced by the ray b' ; the 

 indigo, at I, is a mixture of the rays r', b", b" ; and the violet, at 

 V, is produced by the rays r' and b". There is also in the centre 

 of the solar beam, when the aperture in the window is not too 

 small, a colourless or direct ray, which cannot enter the spectrum, 

 it is always lost in taking the straight direction, m. 



This theory of the prismatic spectrum also scientifically ex- 

 plains why it is an i-iverted image; and, on the legitimate prin- 

 ciples of optics, accounts for the diversity of the spaces which 

 the different colours occupy in it, which cannot be done upon 

 the received hypothesis. Newton supposes the rays to cross 

 before, instead of ofter, they enter the aperture through which 

 they are transmitted. The heating power, and various che- 

 mical effects produced by the prismatic light, may be resolved 

 into three causes :— the difference of polarity in the rays, the 



difference 



