OPTICS. 



recomposed from its elements, is called 

 CHROMATICS, from a Greek word, which 

 signifies colour. 



If we were required to decompose a 



treeiiish grey powder, consisting of fine 

 rass filings, and fine steel filings, we 

 could easily do it by putting into the 

 powder a loadstone, which would in- 

 stantly attract all the blue steel filings, 

 and leave behind the yellow brass filings, 

 thus decomposing, as it were, the green- 

 ish grey powder into the yellow and 

 blue powders, of which it consists. If 

 the steel filings were all much smaller 

 than the bra** ones, we could have se- 

 parated them still easier, by a sieve, 

 whose meshes or holes would permit all 

 the steel filings to pass, while they re- 

 tained the brass ones. 



The decomposition of light is effected 

 in many cases, by processes somewhat 

 analogous. In the experiments on the 

 refraction of water t which are described 

 in Chap. II., and illustrated in fig. 2, we 

 used white light, and we found its re- 

 fractive power, which is represented by 



the line" A D, fig. 2, to be 1 .336. If we 

 now repeat the same experiment, and 

 make the light which passes down the 

 tube, red, orange, yellow, green, blue, 

 indigo, and violet, in succession, we 

 shall find that each colour has a different 

 refractive power of its own, that of the 

 red being the least, and that of the violet 

 the greatest. The following will be the 

 results with water, crown, and flint glass : 



Hence we may conclude, that the rays 

 of light of different colours have different 

 degrees of refraction, or, as it is express- 

 ed, different refrangibilities. 



The method by which Sir Isaac New- 

 ton established this great discovery 

 is shewn in fig. 29. 



In the window shutter E G of a dark 

 room, he made a hole, F, about one- 

 third of an inch broad, and he placed 

 behind it a glass prism A B C, so that the 

 beam of the sun's light SF might enter 

 and leave the prism at equal angles as 

 described in page 8, col. 1. 



Before the interposition of the prism, 

 the beam S F proceeded in a straight 

 line to Y, where it formed a round 

 white spot ; but being now refracted by 

 the prism, it will form upon the screen 

 M N an oblong image P T, containing 

 seven colours, as enumerated in the 

 figure, the red being least, and the vio- 

 let most refracted from the original di- 

 rection S Y of the solar beam. This 



oblong image is called the solar, and 

 sometimes the prismatic spectrum. By 

 making a hole in the screen M N oppo- 

 site any one of these colours, so as to 

 allow it alone to pass ; and by letting the 

 colour thus separated fall upon a second 

 prism, Sir Isaac found that the light of 

 each of the colours was alike refrangible, 

 because the second prism could not sepa- 

 rate them into an oblong image, or into 

 any other colour. Hence he called all 

 the seven colours simple or homogene- 

 ous, in opposition to white light, which 

 he called compound, or heterogeneous. 



With the prism used by Sir Isaac, 

 he found the lengths of the colours and 

 spaces to be as follows : Red, 45 ; 



