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A POPULAR ACCOUNT 



rious other means, such as projecting 

 several spectra produced by different 

 prisms, on the same part of the same 

 paper, by moving several spectra ra- 

 pidly up and down, &c. &c. In all 

 these cases the colours submitted to ex- 

 periment were, however, prismatic. To 

 establish his theory more completely, 

 Newton now proceeds to inquire whether 

 the colours of natural bodies were en- 

 dued with qualities similar in all respects 

 to those of prismatic light. To accom- 

 plish this, having procured powders of 

 colours similar to those of the spectrum, 

 he mixed them together as nearly as 

 possible in the proportion which they 

 were found to hold in the spectrum. 

 He found that the mixture was not a 

 pure white, such as that produced by 

 the composition of the prismatic colours, 

 but was a dim, greyish white ; such, in 

 fact, as would be produced by mixing a 

 small quantity of black with a pure 

 brilliant white. 



(45.) It was not difficult, to account 

 for this circumstance, which Newton 

 appears even to have foreseen. The co- 

 lours of natural bodies arise from a qua- 

 lity, in virtue of which they reflect one 

 component part of the solar beam more 

 copiously than the others, and therefore 

 affect the sense of sight with the colour 

 so reflected. Thus a body which we 

 call red, is one which reflects a "very 

 large portion of the red light of the solar 

 beam, and absorbs nearly the whole of 

 the other six colours. But it is found 

 that no body reflects "the light of its 

 proper colour so copiously as a white 

 body would reflect the same light. If a 

 white and a red object be placed beside 

 each other in a dark room, and both be 

 illuminated with red homogeneous light, 

 by means of a prism, the white object 

 will be more intensely red than the 

 red one. 



(46.) Since then coloured bodies do not 

 any of them reflect all the light of their 

 proper colour, we are not to expect by 

 their mixture to obtain a clear white, 

 but rather such an obscure white as 

 would result from imperfect illumina- 

 tion. That the colour produced by mix- 

 ing powders in the manner already men- 

 tioned is exactly of this kind, Newton 

 proved by the following ingenious ex- 

 periment. 



He placed the mixture of powders on 

 the floor of the chamber, and beside it a 

 piece of white paper. The room being 

 darkened, a beam of light was admitted, 

 so as to illuminate intensely the powder, 



the white paper remaining near it, but in 

 the shade. Viewing them from a dis- 

 tance, he could perceive no difference, 

 both appearing to have exactly the same 

 whiteness. Another person happened to 

 enter the room during the experiment, 

 and Newton, without informing him of 

 the previous arrangement, asked him, 

 "Which of the two whites were the bet- 

 ter, and in what they differed ?" After he 

 had deliberately viewed them, he an- 

 swered, " That both were good whites, 

 that he could not say which was better, 

 nor wherein they differed." Thus it was 

 evident that the colour produced by the 

 mixture of the powders was a true white, 

 but only deficient in the degree of white- 

 ness ; just as twilight is as true alight as 

 broad sunshine, differing from it only in 

 quantity. 



(47.) Having established the important 

 fact, that white must result from the mix- 

 ture of all the colours of the spectrum in 

 the proper proportions, Newton proceeds 

 to the consideration of the more general 

 question a to the colour which would 

 result from the composition of any given 

 colours in any assigned proportion. For 

 this problem he gives the following very 

 ingenious solution. 



With the centre O, fig. 38, and a 

 radius O D, describe a circle AD F, and 

 let the circumference of this circle be 

 divided into 447 equal parts. Take A B, 



Fig. 38. 



consisting of 80 parts, BC of 45, CD 

 of 72, DEofSO, EFof45, FGof45, 

 and the remaining part, G A, will conse- 

 quently consist of 80 parts. Let the first 

 part AB represent a red colour; the 

 second B C an orange; the third C D a 

 yellow, and so on in the order of the 

 spectrum. Let it be conceived that 

 these are all the colours of uncom- 

 pounded light gradually passing one into 



