Composition of White Light. 95 



any subsequent reflexions, multiples of three, a single image, 

 by superposition, at all distances from the prism. 



But, if we suffer a ray of white light, E F {Fig. 4) to 

 fall in the same direction upon the point F, only one of the 

 three colours of which it is composed can pass through the 

 prism parallel to A B ; the others will be dispersed, sup- 

 posing g to be the green ray, the violet to 1 r, and the red 

 to r. 



The angle of dispersion at the internal surface B C is only 

 half what it would be upon emergence, after a second re- 

 fraction, and too small to be correctly represented ; we may, 

 however, trace the different reflexions upon an assumed 

 scale, as in the figure, so as to give an idea of their relative 

 positions, and we shall thus find that the violet ray will 

 emerge, after three reflexions, below the green ray, at an 

 angle with a line parallel to it, equal to the angle of disper- 

 sion, but diverging from it in an opposite direction ; and 

 that, after six reflexions, it will emerge parallel to the 

 direct violet ray, but above it. 



Now, as there must be some breadth of light to form images 

 of the different colours, and, as the space between 1 v and 

 4 v, though differently represented in the figure, is, in fact, 

 very small ; if the light emerged at the same angle it would 

 form only one elongated image; but, by its emission at 

 different angles, the divergence of the direct and the re- 

 flected images is continued after their emergence, and at a 

 sufficient distance, they must be completely separated. 



The chief, if not the only difficulty that presents itself in 

 this arrangement, is, that the reflected violet ray, 4 v, being 

 nearly parallel with the red ray, r, ought, it might be sup- 

 posed, to blend with it in preference to the green ray, g, but 

 this is a difficulty in appearance only ; for, supposing the 

 rays 4 v and r to be parallel, and, consequently, to have the 

 same angle of incidence upon the surface B C, the difference 

 of refrangibility in violet and red light, would cause a rapid 

 divergence of these rays after a second refraction ; and they 

 must, therefore, form distinct images, at a short distance, 

 after their emergence : this being the case, and it being 

 equally impossible that the ray 4 v can blend with the ray 

 ] v, it must either form a junction with the intermediate 

 green ray, g, or emerge separat<'<l ('nun both ; the latter 



