1814.] Experiments on Light. 279 



shining very powerfully ; and liaving made a spectrum, I slowly 

 turned it on its axis, until 1 separated the red and yellow from the 

 blue J and in place of green, white light passed through between 

 the angles. I now ascertained that the red and yellow rays passed 

 through the upper angular edge, by intercepting them with ray 

 finger placed on it ; and by running my finger along the middle 

 angle, 1 intercepted the blue rays ; and by pasting a strip of paper 

 between those two angles, I made two spectra. But to place the 

 fact beyond the possibility of doubt, standing at a little distance 1 

 looked, by means of another prism, at the light passing through, 

 and perceived three beautiful fringes hanging from the angles. 

 Indeed it is surprising that those fringes, as I have proved, so 

 evident to the eye, and so highly important in their consequences, 

 should have escaped the observation of such able and accurate 

 experimenters as those already mentioned. I shall conclude this 

 paper with the following deductions. 



1. Ihat incident light has never yet been decomposed, and that 

 Sir Isaac Newton and other philosophers only decomposed light 

 reflected from opaque substances, Qr fringe* of blue, red, and 

 yellow. 



2. That there are but three primary colours, blue, red, and 

 yellow; by the mixture of which, either by the prism or the 

 painter, all the others are formed. 



3. That Herschel, Leslie, Davy, Englefield, and other philoso- 

 phers, drew their conclusions relative to the heating power of the 

 prismatic colours from erroneous diUn, viz. from experiments on 

 reflected light, whose heal must in a great measure depend on the 

 reflecting media, and also on tlie thickness or thinness of those parts 

 of the prism through which the friijges pass : thus the red and 

 yellow rays passing through the very thin upper angle, must be 

 accompanied bv more radiant caloric than the blue rays which pass 

 through the thickest. The follov\ing diagram will demonstrate mf 

 opinions ; and as I am at present engaged in a series of experiments 

 to prove that the prismatic coloured rays have sin)ilar heatinij 

 powers, I shall not anticipate. 



Let S represent the sun, 

 d, £», /j rays of undecom- ^■••.(s) 



jx)sed light, impinging on :::--''"^>'" 



the angles, A, B, C, of the i"^;^"^^ 



jjrism; these carry forward 1 tl^«w"' 



the angular fringes, which | H \u;t^ui;gh 

 b«ing refracted towards the I i 



perpendicular, fall on the ^;j,:^^"mac 



spectrum, IJ. The red 



and yellow rays passing through the thin angle, A, must he mor« 

 heated when falling on the spectrum, II, than the blue ray 

 posing through the angles, 15, C. 



^ir, I beg leave to icuiain your obedient servant, 



