38 



A POPULAR ACCOUNT 



distinct ; and the liquid in each cylinder 

 would present, in the vertical direction, 

 an uniform colour. 



(48.) Connected with the power of trans- 

 parent liquids to reflect and transmit the 

 different component parts of solar light, 

 Newton mentions two very remarkable 

 facts noticed by Halley and Hooke, but 

 which these eminent philosophers were 

 unable to explain. Halley, having de- 

 scended in a diving bell to the depth of 

 several fathoms in the sea, observed, 

 upon holding his hand in the sun's light, 

 which penetrated the water, and shone 

 into the bell through a small glass win- 

 dow in the top, that the light upon his 

 hand was red. Whereupon he examined 

 the lower part of his hand illuminated by 

 light reflected from the water below, and 

 found it green. This circumstance is 

 thus accounted for by Newton. The 

 sea-water reflects back the violet and 

 blue rays most easily, and transmits most 

 copiously the red. In the sun's light 

 transmitted to considerable depths, the 

 red rays therefore predominating, objects 

 illuminated by them assume a red hue. 

 At depths to which the violet rays cannot 

 penetrate, the reflection of the blue, 

 green, and yellow light separated from 

 the red, which is transmitted, must com- 

 pound a green. 



Two liquids may be obtained, one of 

 which transmits the rays of the red 

 character, and the other those of the 

 blue, the former intercepting the bluish 

 light, and the latter the red. If both 

 liquids be placed between a spectator and 

 the light, they will be found perfectly 

 opaque, although either alone is trans- 

 parent. This is evident, since all the 

 rays which can be transmitted by either 

 are intercepted by the other. Hooke 

 casually, and without anticipating or ex- 

 pecting the result, actually tried this ex- 

 periment. He filled two hollow glass 

 wedges, one with a red, and the other 

 with a blue liquor. On placing the 

 wedges together, and looking through 

 them at the light, he found them abso- 

 lutely opaque. 



(49.) We have explained, according to 

 the Newtonian theory, the most striking 

 phenomena of coloured lights produced 

 by prisms. The explication of others 

 will be found in every elementary treatise 



on optics. One very singular prismatic 

 phenomenon, however, still remains to be 



noticed, and is entitled to attention, as 



well for the strong confirmation of New- 

 ton's theory which it furnishes, as from 



the ingenious manner in which that 

 theory is shown to account for it. 



Let HKG, fig. 39, be a prism placed be- 

 fore an open window, with its base H E I G 



Fig. 39. 



horizontal, the face F K G I presented 

 to the light of the clouds, and let the 

 base be viewed through the face F K H E 

 by an eye at S. The" base H E I G will 

 now be observed to be separated into two 

 parts by a beautiful iridescent arch, 

 formed of colours of violet and bluish 

 tints. This arch is concave towards the 

 eye, and that part of the base which is 

 towards the edge I G, or above the arch, 

 exhibits a most vivid reflection of the 

 firmament, not yielding in splendour to 

 the direct view- of the heavens. On the 

 other hand, the lower division of the 

 base next the edge E H, appears nearly 

 dark, reflecting but a very small portion 

 of the light incident upon it. The arch 

 next this sombre space is fringed with a 

 violet colour, which is gradually tinted 

 off into a vivid blue towards the convex 

 edge, which bounds the bright part of the 

 base. 



To account for this curious phenomenon, 

 it must be remembered that the different 

 parts of solar light are differently re- 

 flexible ; also, that when rays of light are 

 incident on the base of a prism, having 

 previously passed through its side, there 

 are certain angles of obliquity at which 

 it will be impossible for the rays to pass 

 through the base, and they will then be 

 reflected. The limit of obliquity at which 

 they will cease to penetrate the base, and 

 will be reflected, depends on their degree 

 of refrangibility. The most refrangible, 

 and consequently the most reflexible rays, 

 are the violet, next to these the indigo, 

 then the blue, and so on through the 

 other colours of the spectrum, the red 

 being least reflexible. Let H E I G be 



