LAW OF DISPERSION BY REFRACTION. C5 



cumstancc is, that he appears, like many persons in whom the poetical 

 imagination is very active, to have been destitute of the talent and 



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the habit of geometrical thought. In all probability, he never appre- 

 hended clearly and steadily those relations of position on which the 

 Newtonian doctrine depends. Another cause of his inability to accept 

 the doctrine probably was, that he had conceived the " composition " 

 of colors in some way altogether different from that which Newton 

 understands by composition. What Guthe expected to see, we cannot 

 clearly collect ; but we know, from his own statement, that his inten- 

 tion of experimenting with a prism arose from his speculations on the 

 rules of coloring in pictures ; and we can easily see that any notion 

 of the composition of colors which such researches would suggest, 

 would require to be laid aside, before he could understand Newton's 

 theory of the composition of light. 



Other objections to Newton's theory, of a kind very different, have 

 been recently made by that eminent master of optical science, Sir 

 David Brewster. He contests Newton's opinion, that the colored rays 

 into which light is separated by refraction are altogether simple and 

 homogeneous, and incapable of being further analysed and modified. 

 For he finds that by passing such rays through colored media (as blue 

 glass for instance), they are not only absorbed and transmitted in very 

 various decrees, but that some of them have their color altered ; which 



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effect he conceives as a further analysis of the rays, one component 

 color beino- absorbed and the other transmitted. 22 And on this sub- 



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ject we can only say, as we have before said, that Newton has incon- 

 testably and completely established his doctrine, so far as analysis and 

 decomposition by refraction are concerned ; but that with regard to 

 any other analysis, which absorbing media or other agents may pro- 

 duce, we have no right from his experiments to assert, that the colors 

 of the spectrum are incapable of such decomposition. The whole sub- 

 ject of the colors of objects, both opake and transparent, is still in 

 obscurity. Newton's conjectures concerning the causes of the colors 

 of natural bodies, appear to help us little ; and his opinions on that 

 subject are to be separated altogether from the important step which 

 he made in optical science, by the establishment of the true doctrine 

 of refractive dispersion. 



[2nd Ed.] [After a careful re-consideration of Sir D. Brcwstcr's 

 asserted analysis of the solar light into three colors by means of 



22 This latter fact has, however, been denied by other experimenters. 

 VOL. II. 5. 



