^94 Considerations on Colours, 



His means of making experiments are simple. If the 

 body be opake, it is placed on a piece of black stuff in order 

 to be observed with the prism. He endeavours to give it a 

 rectangular form ; or, if it is not susceptible of being cut, 

 it is covered with a piece of black pasteboard pierced with 

 a hole of that form. The coloured fringes, then, manifested 

 on the two opposite edges indicate the kind of ravs which 

 are reflected, and consequently those which arc absorbed 

 when the nature of the illuminating bundle is known ; on 

 which it is still to be remarked that, as the fringes them- 

 selves are of complex shades, we must separate the simple 

 kinds. When a person has had some practice, a bare in- 

 spection will be sufficient. He may be formed to this habit, 

 and the waut of it may be supplied by guiding himself by 

 cards representing each kind of rays placed over each other 

 in order, removing them gradually agreeably to the different 

 refrangibility ; or he may employ a plate or board, con- 

 structed according to Newton's method, for determining 

 the shades composed of different elementary colours. 



If the body subjected to examination be diaphanous, it 

 W'ill be proper to view it through the aperture of the card 

 before spoken of, in order to exclude extraneous light, in 

 such a manner that the prism may show the fringes. Also, 

 by placing yourself in the dark, a flame such as that of a 

 wax taper w-ill show through the transparent body, and, 

 by the help of the prism, a series of coloured images corre- 

 sponding to the rays transmitted. 



By proceeding in this manner, the author found that 

 many opake bodies which he had at hand of different kinds 

 and of all colours, either yellow, orange, or red ; or green, 

 blue, or violet, were indebted for their coloured appearance 

 to the following conditions : 



1st, Each of the bodies always absorbs rays of the com- 

 plementary kind of the prevailing colour. 



2d, The absorption, in regard to some of them, compre- 

 hends, besides the complementary kind, other rays colla- 

 teral to that species, and more or less numerous. 



3d, The darker the same colour is, the fewer kinds of 

 reflected rays it presents. 



It must here be understood that we do not allndc to 

 mixed colours, but only to those w hich form a homoge- 

 neous compound or a real comlwation, according to the 

 meaning attached by chemists to that word. It is also to 

 be remarked, that we must not confound the colour reflected 

 from the interior of the moleculse susceptible of bright or 

 dark shades with the light sent back from the anterior sur- 

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