164 On the Decomposition of Light. 



4th, Lastly, if we look at a white and narrow body placed 

 upon a black ground, such as a strip of paper^ a thread of 

 silk, or a metallic needle, there will only appear, for one 

 of ttiese objects, if we are properly placed, three colours, 

 red, gieen, and violet; and scarcely can we perceive either 

 in the one or the other any softening of the shades. 



These various phaenomena are incompatible with the sup- 

 pcjsition of an uniform gradation of colours throughout the 

 whole length of the spectrum, that is to say, incompatible 

 with a variation of strength, owing to one sole and uniform 

 law; but thev do not contradict the results of the experi- 

 ment which gare to Newton seven classes of particular co- 

 lours. If, in some cases, as we shall soon see, refrangi- 

 bility reduces the apparent colours to four principal kinds, 

 or to three, or at other times shows them in a state more or 

 less confused, it is an irresistible consequence of the relation 

 existing between the breadth of the body observed, the angle 

 of dispersion of the rays, and the distance at which the 

 image is received. 



We may, by means of a figure, account for these effects : 

 it results from this, that, in order to obtain by refraction the 

 most colours when the rays are primitively parallel, there 

 must be a very small luminous object, a considerable angle 

 of dispersion, and a very great removal of the image : hence 

 the difficulty of repeating with success the experiment of 

 Newton, where he proposes to disengage as much as pos- 

 sible the heterogeneous rays from each other*. 



This we may also infer from the very interesting experi- 

 ments of M. Rochon upon the light of the fixed stars f. 

 This philosopher undertook these examinations on account 

 of the advantage of the smallness of the apparent diameter 

 of these stars; and in order to attain his object, he inge- 

 niously arranged oefore an excellent telescope an achromatic 

 prism, of his own invention, with a variable angle : with 

 this apparatus, and the refracting angle of his prism being 

 14^ orxly, he saw among others the star Sirius, the spec- 

 trum of which was almost entirely composed of three co- 



* Optics, book i. part 1. exper. xi. 

 f Recucil de Memuires, p. J:). 



lours ; 



i 



