468 Sir David Brewster on the Colours of Natural Bodies. 
of the rays of the solar spectrum, and § the index of refrac- 
tion of this ray passing from the air-into another medium: it 
follows from the principles established in my memoir on the 
dispersion of light, that we may develop in a converging series 
(+) é according to the ascending powers of (+) ‘ and con- 
0.” ; 
sequently ae, and §° according to the ascending powers 
erp) - Moreover, a very able observer, Fraunhofer, has de- 
termined, in respect to different substances, the indices of 
refraction of the rays for which the values of 7 in hundred- 
millionth parts of an inch, are 
2541, 2425, 2175, 1943, 1789, 1585, 1451, 
and finds as the corresponding values of § relative to a cer- 
tain species of flint-glass, 1°626469; 1°628469; 1:633667; 
1°640495; 1°646756; 1°658848; 1°669686: in which case 
the formula (VIII.) being then reduced to 
2 4 
6? = 2°6112351 —0:0256298 (4) +0:1081567 (4 
Ye 
0:0649226 etal +0°019115 ear oe: 
ai ; (+-) —0-002189 (—*) ; 
reproduces exactly, and without the slightest alteration, the 
preceding values of 6. 
September 1835. 
LXXIX. On the Colours of Natural Bodies. By Sir Davip 
Brewster, K.H. LL.D. F.R.S. Lond. V.P.R.S. Edin.* 
HERE are few of the applications of optical science so 
universally interesting as that which has for its object the 
explanation of the colours of natural bodies. Sir Isaac New- 
ton was the first person who ventured to refer to one general 
principle all the variety of colours which are found in nature ; 
and he maintained his opinions on this subject with a con- 
fidence in their accuracy which seems to have confounded his 
adversaries: For while his analysis of light, the most perfect 
of all his labours, exposed him to the most harassing contro- 
versies, his theory of natural colours, the least perfect of his 
speculations, was allowed to pass without examination or 
censure. | 
During the century which has elapsed since the death of 
* From the Transactions of the Royal Society of Edinburgh, an unim- 
portant paragraph being omitted. 
