32 



A POPULAR ACCOUNT 



the other hand, a body whose natural 

 colour is red, illuminated in the same 

 way with blue light, will appear blue. 



(40.) In the theory derived by Newton 

 from the experiments which have been 

 explained, the white light of the sun is 

 supposed to be compounded of several 

 component lights which have qualities 

 different each from the others. They 

 are all refrangible according to the same 

 law discovered by Snellius (10); but, 

 as we have already shown, they possess 

 this quality in different degrees. This 

 property is accompanied by another in- 



timately connected with it. Any two of 

 the component parts of solar light which 

 differ in refrangibility, differ also in 

 colour ; and therefore the light of the 

 sun is composed of various species of 

 light of different colours, the mixture of 

 which produces whiteness. 



Newton next proceeds to determine 

 the degrees of refrangibility correspond- 

 ing to the rays of different colours. To 

 determine this by experiment, he de- 

 lineated, on a paper, the outline of the 

 spectrum, fig. 33, F A P G M T, and 

 refracting the sun's light by a prism, as 



Fig. 33. 



described in p. 15, he held the paper 

 so that the spectrum might exactly fall 

 upon the space marked out upon it. 

 He employed an assistant, whose per- 

 ception of colours he considered to be 

 better than his own, who drew lines 

 across the paper, marking the confines 

 of the several colours. Thus ab divided 

 the red from the orange; c d, the orange 

 from the yellow; ef, the yellow from the 

 green ; g h, the green from the blue ; i k, 

 the blue from the indigo ; Im, the indigo 

 from the violet. This experiment was 

 frequently repeated on the same, as 

 well as on different papers, and the 

 results were found to be generally ac- 

 cordant. Let G M be drawn to X, so 

 that MX shall be equal toGM, the 

 spaces measured from G to the several 

 boundaries of the colours were found to 

 have the following proportion : 

 XG, XI, X?', Xg, Xe, Xc, Xa, XM, 



1, I, f. i> > f> TV ** 

 The spaces measured along the spec- 

 trum occupied by the lights of the several 

 colours may be considered to measure 

 the differences of the sines of refraction 

 of those rays having one common sine 

 of incidence. But the proportion of 

 the sine of incidence to that of refraction 

 from glass into air has been already as- 

 certained to be 50 to 77 for the least, 

 and 50 to 78 for the most refrangible 

 rays ; it follows, therefore, that if 50 be 

 the common sine of incidence, the sines 

 of refraction for the rays at the bounda- 

 ries of the several colours, beginning 



* The analogy observed by Newton between the 

 proportion of these intervals and the musical inter- 

 vals must be regarded as merely fanciful, 



from the red, will be 77$, 77 J, 77^, 77^, 

 77f, 77JJ, 78, which may be familiarly 

 explained thus. Let A B, fig. 34, be 



Fig. 34. 



s 



the surface of glass from which the ray 

 S I passes at the point I into air. Let 

 the ray S I be solar light. Round the 

 point I as centre describe a circle, and 

 through I draw a diameter E F perpen- 

 dicular to the refracting surface A B. 

 From the point C, where the ray meets 

 this circle, draw C D : this is the sine 

 of incidence. Let it be divided into 50 

 equal parts. Upon I B from 1 take a 

 length lr' equal to 77 such parts, and 

 draw r'r perpendicular to I B ; again, 

 take I o', equal to 77$ of those parts, and 

 draw o'o perpendicular to I B. In the 

 same manner, take I y f , I g', I b', I i', 

 Iv', equal to 77$, 77, 77$, 77|, 77|, 78 

 parts respectively, and draw, as before, 



ry, g'g, b'b, i'i, o'o. From I draw I r t 

 o, ly, Ig, Ib, It, Iv. These lines 

 will determine the directions of the red, 

 orange, and the other rays corresponding 



