Permanent Colours of Opake Bodies. 175 



In making thefe obfcrvations, it is proper to 

 choofc coloured plates of glafs, which are not in 

 every part of an equal thicknefs, that the fecon- 

 dary image may not be relieved in fuch a direc- 

 tion, as to coincide v/ith, and be intercepted by, 

 that which is refleded from the anterior furface. 



When the plates are fo thick, and fo copioufly 

 coloured, that the light cannot penetrate to their 

 farther furface, they appear intenfely black, in 

 whatever direction they are viewed, and they do- 

 not afford any fecondary image; but only refledb 

 from their anterior furface, the flame, or any 

 other objefts which are oppofed to them. Thefe 

 objefls are reprefented in their own proper co- 

 lours, and are as free from any tinge, or adven- 

 titious colour, as thofe which are refledled from 

 looking glaffes, or fpecula made of white metals. 



It is manifefl from hence, that the colouring 

 particles have no (hare in effecting this refieflion. 

 For, if they poffcffed any reflexive power, 

 whence is it that glafles, copioufly ftored with 

 colouring matter, impart no colours to the 

 -imases, which they re fled ? 



It appears, from thefe obfcrvations, that tranf- 

 parent coloured bodies, as well in a folid, as in 

 a fluid, ftate, exhibit colours by tranfmiffion only, 

 but do not refled any colour. 



The order of this inquiry leads me, in the 

 next place, to the confideration of the Colouring 

 Particles, pure, and unmixed with other media. 



For 



