AND ON THE ABSORPTION OF THE PRISMATIC COLOURS. 441 



green colour, it passed through various shades of olive-green ; 

 but its tint, when cold, continued less green than it did before 

 the experiment. A part of the glass had received in cooling 

 a polarising structure, and this part could be easily distinguish- 

 ed from the other part by a difference of tint. 



A plate of deep-red glass, which gave a homogeneous red 

 image of the candle, became very opaque when heated, and 

 scarcely transmitted the light of the candle after its red heat 

 had subsided. It recovered, however, its transparency to a 

 certain degree, but when cold, it was more opaque than the 

 piece from which it was broken. 



3. As it has been concluded from Dr Wollaston's experi- 

 ments on the prismatic spectrum, that it consists only of four co- 

 lours, Red, Green, Blue and Violet, that pure white light does not 

 contain any yellow rays ; — and that when yellow does appear 

 at the boundary of a large white space seen through the prism, 

 it is a compound light, consisting of Green and Red rays *, it oc- 

 curred to me that the accuracy of these opinions might be ex- 

 amined by means of the absorbing media already mentioned. 



I therefore formed a very brilliant spectrum, from a narrow 

 aperture placed between my eye and the sun, and by varying 

 the distance of the prism from the aperture, I obtained a spec- 

 trum exactly similar to that described by Dr Wollaston and 

 Dr Young, who has adopted and illustrated the opinions of his 

 friend respecting the composition of white light. 



Upon viewing this spectrum through a glass, which absorbed 

 the Red next the Green, and also through a glass which ab- 

 sorbed the Green next the Red, I was entitled to expect that 

 these portions of the spectrum would vanish ; but instead of 

 this taking place, the space from which these colours were ab- 

 sorbed was in both cases occupied by Yellow light, which was 



3 k 2 not 



* Dr Thomas Young's Lectures on Natural Philosophy, vol. i. p. 138. 



