24 



OPTICS. 



orange, 27 ; yellow, 40 ; green, 60 ; blue, 

 60 ; indigo, 48 ; violet, 80 : or 360 in all. 



But these spaces vary with prisms of 

 different substances, and as they are 

 not separated by distinct limits, but 

 shade gradually into one another, it is 

 almost impossible to obtain any thing 

 like an accurate measure of their rela- 

 tive extents. This difficulty is increased 

 too by the circumstance, that as the 

 spectrum is brightest in the yellow 

 space, and grows fainter and fainter 

 towards the red and the violet extremi- 

 ties, its length increases with the inten- 

 sily of the light from which it is formed. 



Having thus decomposed white light 

 into its seven primary colours, Sir Isaac 

 Newton shewed, that these seven co- 

 lours, when again put together, or com- 

 bined, recomposed white light. This 

 may be proved rudely, but yet accu- 

 rately enough for the purposes of illus- 

 tration, by mixing together seven diffe- 

 rent powders, having the colours and 

 proportions indicated above ; or, what is 

 better, by painting the rirn of a wheel 

 with the seven prismatic colours, and 

 making it revolve rapidly about its axis. 

 In both these cases the mixture of the 

 colours will be a sort of greyish white, 

 because the colours employed cannot 

 possibly be obtained of the proper 

 tints, or laid on in the proper propor- 

 tions. A more accurate proof is obtained 

 by making the prismatic spectrum fall 

 on a lens or concave mirror, and thus 

 bringing the whole seven colours into a 

 focus, which will be white 



CHAPTER IX. CHROMATICS con- 

 tinued. 



Dispersion of Light Dispersive Pow- 

 ers Table of Dispersive Powers. 



IN the prismatic spectrum P T, 

 formed by the prism ABC, (fig. 29), 

 the green space G is placed^ in the 

 middle between P and T; and hence 

 it has been called the mean ray of the 

 spectrum: the index of refraction, 

 which belongs to it, is called the mean 

 refractive power of the prism ; and the 

 angle, which the green ray forms with 

 the line S Y, the mean refraction of the 

 prism. 



Although Sir Isaac Newton seems to 

 have made use of prisms of different sub- 

 stances, yet it is strange to say, that he 

 never observed that they formed spec- 

 tra, whose lengths PT were different, 

 when the mean refraction of the green 

 ray was the same. If, for example, we 

 make a prism of plates of glass, and 

 fill it with oil of cassia, and adjust its 

 refracting angle A C B, so that the mid- 

 dle of the spectrum, which it forms, 

 falls exactly on the point G, where the 

 green space is with the glass prism, 

 then we shall find, that the spectrum 

 of the oil of cassia prism will be two 

 or three times longer than that of the 

 glass prism ; the oil of cassia is therefore 

 said to disperse the rays of light more 

 than the glass, that is, to separate the 

 extreme red and violet rays at T and P 

 more from the mean ray G, and to have 

 a greater dispersive power. 



In order to obtain a distinct measure 

 of the dispersive power of a body, let 

 us suppose that the prism ABC is 

 filled with water, and that by the me- 

 thods described in Chap. II. we find the 

 index of refraction for the extreme vio- 

 let ray P to be 1.330, and that of the 

 extreme red ray T, 1.342 ; then the dif- 

 ference of these, or 0.012, would be a 

 measure of the dispersive power of 

 water, if it and all other bodies had the 

 same mean refraction ; but as this is 

 not the case, the dispersive power must 

 be measured by the relation between the 

 separation of the extremes rays P, T, 

 and the mean refraction; or between 

 the difference of the indices of refraction 

 for the extreme red and the extreme vio- 

 let, and the difference between the Sines 

 of incidence and refraction, to which 

 the mean refraction is always propor- 

 tional. 



Thus, in diamond, the difference be- 

 tween the indices of the red and violet 

 ray is 0.056, nearly five times greater 

 than 0.012 which it is in water ; but then 

 the difference between the Sines of inci- 

 dence and refraction, viz. 1.439, is also 

 nearly five times greater than 0.336, 

 which it is in water ; so that the real 

 dispersive power of diamond is not 

 much greater than that of water. The 

 ratio of the dispersive powers will be 

 thus expressed : 



For Water. . 



'" ' 351 



For Diamond 



or - = . 0388 Dispersive power. 



