and several of their sinpthr Appearances. 295 



face of the body. Though the latter overcharges more or 

 less the proper colour, it is, however, ea&y to lessen the 

 effects ef it, and to distinguish them in exf>enments. 



Another remark proper to be made is, that the expression 

 predominating colour ought not to sigiiiiy that the rays of 

 that colour are more abundant than the rest : this would be 

 an error. Several kinds of rays niay co-exist in the bundle 

 which produces the colour, without any kind being, on 

 that account, more abundant. Strictly speaking, all the 

 elements of the bundle are dissimilar, and consequently 

 none of them is in greater quantity. But the general tone 

 of the colour remains analogous to that of the rays distin- 

 guished by the name oi predumivating. Hence it is proper 

 to retain this expression, provided an exaggerated significa- 

 tion be not given to it. 



The author observed also transparent bodies, such as 

 glass of different colouis, and liquors contained in a flask 

 having two broad parallel faces. By these he found a law 

 of absorption similar to that of opake bodies, but still more 

 striking, and without any ambiguity. 



This law is constantly regular, it depends on the pecu- 

 liar nature of the body which receives the light, and on its 

 densitv and thickness. It is also determined by the light 

 of the illuminating body, either in regard to its force, or to 

 the two kinds of rays which compose it. 



The absorption always begins with tlie rays most opposite 

 to the predominating colour of the ilhniiinated hodv. It 

 continues by those which are next in the order indicated 

 bv the spectrum. It thus extends gradually, and never by 

 jumps, to the last kind : consequently the body becomes 

 more and more obscure, and always terminates by being 

 black. Sometimes it extends from one side only of the 

 tlrst rays absorbed ; sometimes on both sides at the same 

 time ; and it there proceeds either by an equal progress 

 from the right and left, or by advancing more rapidly on 

 one of the sides. 



If each element be separately varied, there will be in the 

 effects a peculiar progression. That depending on the densi- 

 ties is not always similar to that arising from the changes 

 of thickness. Bv receivino: also on the same body different 

 kinds of light, the progress^of the absorption is differently 

 modified, and consequently the colours changed. 



The author quotes examples of all these Oases. He de- 

 rives them Irom the numerous ex}erinn'nts which he made 

 with coloured glass, with acid or alkaline metallic solu- 

 tions, with the liquid tinctures of infusions or vegetable 

 T 4 decoctions. 



