270 Theory of Colours. [Book III. 



parted from the prifm, but in a reverfed order, by the 

 eroding of the rays j the reafon of which is clear ; for 

 the ray being white before it was divided by the prifm, 

 muft neceffarily become fo by the re-union of its part.^ 

 which the difference of refrangibility had feparated, 

 and this re- union cannot in any manner tend to alter 

 or deftroy the nature of the colours j it follows then 

 that they muft appear again beyond the point of crof- 

 fing. 



In the fame manner, if we mix a certain proportion 

 of red colour with orange, yellow, green, blue, indigo, 

 and violet, a colour will be produced which refembles 

 that which is made by mixing a little black with white, 

 and which would be entirely white if fome of the rays 

 were not loft or abforbed by the groflhefs of the co- 

 louring matter. 



A colour nearly approaching to white is alfo formed 

 by colouring a piece of round pafteboard with the dif- 

 ferent prifmatic colours, and caufing it to be turned 

 round fo rapidly that no particular colour can be per- 

 ceived. 



If to a fingle ray of the fun divided by the prifm, 

 which will then form an oblong-coloured fpeftrum, a 

 thick glafs deeply coloured with one of the primitive 

 colours is applied, for example red, the light which 

 pafies through will appear red only, and will form a 

 round image. 



If two thick glalTes, the one red and the other green, 

 are placed one upon another, they will produce a per- 

 fect opacity, though each of them, taken feparately, is 

 tranfparent, bccaufe the one permits the red rays only 

 to pafs through it, and the other only green ones, 

 therefore when thefe two glaffes are united, neither of 

 thofe kind of rays can reach the eye, becaufe the firft 

 permits only red rays to pafs, whereas die fecond re- 

 * ceives 



