1835.] Compositionof White Light. 175 



It will be seen, upon an inspection of the two images, 

 that the accidental image is formed by light reflected from 

 the first surface of the coloured glass, without any admix- 

 ture of coloured light from the mirror ; this part of the 

 mirror being invariably occupied with the shadow of the 

 finger ; the accidental image, therefore, always corresponds 

 with the shadow of the intercepting object, which, by suf- 

 fering it to overlap the coloured glass, will be seen conti- 

 nued upon the mirror in corresponding lines. The coloured 

 image, on the contrary, is regulated by the position of the 

 coloured glass, and is evidently formed by the coloured light 

 reflected by the mirror through that part of the glass, from 

 the first surface, of which there is no reflection, the direct 

 light being intercepted by the finger. The accidental 

 image, then, is formed by the light reflected from the first 

 surface only ; the coloured image is formed by light trans- 

 mitted twice through the coloured glass, without any mix- 

 ture of the light reflected by the first surface ; and the 

 remainder of the glass derives its colour from a mixture of 

 both. Hence it is, that the coloured image is much more 

 brilliant than the general surface, the latter being dieted 

 with the white light reflected by the first surface. The 

 coloured image is beautifully transparent, while the other 

 parts of the surface exhibit a slight degree of opacity, or, 

 what may be termed, a glassy appearance, arising from the 

 first reflection, which is the more conspicuous from its being 

 formed, whatever may be the colour of the reflectors of 

 white light. 



The two images being thus accounted for, we have now 

 to explain how the one becomes the accidental colour of the 

 other. 



We are not to suppose, because the light reflected by the 

 first surface is white, that the image formed by it must be 

 a white, corresponding with the surrounding atmosphere ; 

 the quantity of light thus reflected is so small, that it ought 

 to be invisible to an eye impressed with this light ; and 

 this, in fact, is the case ; the image being frequently seen, 

 when the eye has been previously occupied with strong 

 white light, nearly, or quite black ; ''^ as the eye gradually 



* Mr. Delaval observed that all coloured liquids appeared black by reflected 

 light, when there was no reflection from the second surface. 



