540 Sir DAVID BREWSTER on the Colours of Natural Bodies. 



ally not very full, yet are often too full and lively to be of the 

 fourth order." 



Having thus determined that the green colour of vegetables 

 must, according to this theory, be a green of the third order, we 

 must inquire into its composition. Sir ISAAC has himself stated, 

 that the green of the third order " is principally constituted 

 of original green, but not without a mixture of some blue and 

 yellow." In point of fact, it consists of all the rays of the green 

 space, with the least refrangible rays of the blue space, and the 

 most refrangible rays of the yellow space, and it does not contain 

 a single ray of indigo or violet, nor a single ray of orange or red 

 light. This is its real composition, whether we deduce it from 

 the theory of periodical colours, or obtain it by direct analysis 

 with the prism. 



In order to discover the true composition of the green colour 

 of plants, we may analyze the light which they reflect or trans- 

 mit ; but the best method is to extract the green colouring mat- 

 ter by means of alcohol, and to examine the action of the tin- 

 ging corpuscles when suspended in that fluid. For this purpose 

 I have used the leaves of the common Laurel, Primus Lauro-ce- 

 rasus, as a type of this class of colours. The leaves are torn into 

 small shreds and put into absolute alcohol, and the fine green fluid 

 which is thus obtained is either placed in a hollow prism with 

 a large refracting angle, so as to exhibit its composition in its 

 own spectrum, or the light transmitted through the fluid may be 

 analyzed by a fine prism, or the spectrum produced by such a 

 prism may be viewed through a portion of the fluid bounded by 

 parallel surfaces. By whichsoever of these methods the experi- 

 ment is made, we shall observe a spectrum of the most beautiful 

 kind. In place of seeing the green space with a portion of blue 

 on one side and yellow on the other, as the Newtonian theory 

 would lead us to expect, we perceive a spectrum divided into se- 

 veral coloured bands of unequal breadths, and having their colours 

 greatly changed by absorption. 





