1835.] Composition of White Light. 363 



yellow ; if we then interpose the yellow glass, before spoken 

 of, between the eye and the prism, we shall find that the 

 crimson will be re-converted to red, and that the blue will 

 be changed to green. Now, having previously ascertained, 

 by a variety of experiments, that this crimson is formed by 

 the intersection of the violet and red fringes, we here per- 

 ceive that the very same action of the coloured glass which 

 restores the colour of the red, converts the blue to green. 

 The inference which must be drawn from this experiment, 

 that blue is a compound colour, does not admit of a doubt. 



If the two surfaces of white light be separated by an object 

 of greater breadth than the window bars, the violet light 

 will be more distinctly seen before it intersects the red ; 

 and if in this state of the fringes we interpose the yellow 

 glass, the violet will disappear, and the blue will be con- 

 verted to green as before : it is therefore evident, from both 

 these experiments, that blue is a compound of green and 

 violet. By looking at distant objects, such as houses, trees, 

 &c., this sudden conversion, upon the interposition of the 

 yellow glass, is rendered very striking and beautiful. 



The orange tint is given to yellow glass by its absorption, 

 in addition to nearly the whole of the violet, of a larger 

 proportion of the green than of the red light ; and, upon 

 this principle, by increasing the number of glasses, we may 

 convert the yellow fringes, together with the whole of the 

 white light, to a red. This experiment is not so striking 

 as the former ; because the absorbing power of the glass is 

 not so decided for green as for violet light ; but the prin- 

 ciple is the same, and it proves very clearly, that the same 

 action of the coloured glass which removes the green 

 fringes, converts the yellow to red. 



With crimson glasses, the yellow fringes are converted 

 to a beautiful scarlet. The white light between the fringes, 

 when seen through this glass is crimson, or red and violet ; 

 but there being no violet in the yellow fringes, the red only 

 is admitted. The blue fringes are changed by crimson 

 glasses to violet, by the absorption of the green. 



The experiments with these coloured fringes might be 

 varied so as to produce a body of evidence, which if it stood 

 alone would be sufficient to decide the question ; but in this, 

 as in other instances, I have produced such experiments 



