I5£C0MPOSI*ION OF LIGfHT. 27 



of structure, Avith the finest polish ; in short, it was to all 

 appearance free from defect. 



It now remains for me to speak of -the particular ex-Experiments 

 periments on the analysis of colours, which I had long- 

 planned. 



The reader may recollect, that I had suspected the blue Hypothesis, 

 to be merely the result of a combination, of green and ^et^compTund of 

 rays; and that in like manner the yellow proceeded only' green and 

 from a mixture of green and red. I reasoned then in this ye i Jo V, of 

 manner: on the supposition that in reality there existed no green and red. 

 simple rays of blue, if we prevent the arrival of rays to Consequcnces * 

 that part of the spectrum, either by a substance that-sufiers 

 only the green rays to pass, or by one that allows a passage 

 to the violet only, we shall find beyond these substances 

 only green or -violet ; otherwise, supposing the blue rays to 

 be simple, they will traverse neither of the substances I 

 have mentioned, as we shall find beyond them nothing but 

 black. We may reason in a similar Avay Avith respect to 

 the yelloAv, AVhich must be subjected to the trial of a red 

 substance and a green. 



Thus we must be furnished Avith three substances coloured Requisites for 

 in the requisite manner. For the violet I employed an men V. 

 ammoniacal solution of copper, in a phial Avith plane 

 parallel surfaces : for the green a solution of muriat of 

 copper, in a similar phial: and for the red, either Avine of 

 a good colour, or a tincture of cochineal. All these must 

 be sufficiently concentrated, or they will transmit other 

 rays, beside those we have in view. This concentration 

 has the inconvenience of rendering the colour obscure, it is 

 true, and this is some obstacle to their use; but it is the 

 only way in which nature permits us to obtain simple 

 colours, and we must be content with it. 



Coloured glasses might be substituted for the red and 

 green liquors; but with respect to the violet I could not 

 procure any, on Avhich I could depend. 



Every thing being thus prepared, I made my experiments 

 in concert with Mr. Trcmer} and Mr. Drappier. 



We had a screen, which avc could place at will before Apparatus. 

 the cloth opposite the place of (he spectrum. In this screen 

 was a small ckcular hole 2 or 4 millimetres (I. 17 or I. 56 



line) 



