278 Experiments on Light. [April, 



and yellow : paste another similar strip parallel to this, at about four 

 inches distance, on looking through the plism you perceive the 

 north to be fringed with blue : thus we have three primary colours 

 nearly in contact ; the yellow rays of tlie upper paper being the 

 most refrangible come nearest to the blue of the lower paper ; and 

 if we approach them, a green is formed by their mixture ; so that 

 we can now without any difficulty account for five of Sir Isaac 

 Newton's colours — red, orange, yellow, green, and blue. By making 

 a small hole in his window-shutter, he brought the northern and 

 southern fringes into contact or mixture, and produced five colours 

 with three. It now remains to account for the indigo and violet : 

 and here I must again refer my reader to my last communication, 

 in which I have shown that blackness arises from the reflection of 

 blue, red, and yellow ; which being granted, the solution of this 

 Otherwise difficult question becomes easy. The red and yellow of 

 the lower cloth or paper being more refrangible than the blue, were 

 brought down by passing through the prisn), leaving the upper part 

 of the lower edge (when illuminated by the undecomposed light 

 coming through) blue; under the blue appeared indigo, which, as 

 I shall hereafter show, is composed of blue, red, and yellow, in a 

 different state of condensation from black : and at the bottom of all 

 appeared violet, arising from a great quantity of yellow and red 

 which had been brought down mixed with the black rays. From 

 this experiment we might conclude that Sir Isaac Newton, by mixing 

 three primary colours, made seven. 



But I am well aware it might be objected that Sir H. Englefield 

 and others decomposed, or thought they decomposed, incident light 

 coming immediately from tlie sun, by passing it through a prism 

 placed at an open window : so far, liowever, from refuting, this 

 experiment confirms my opinion that incident light was never yet 

 decomposed, as I shall now endeavour to prove. The prism being 

 a semitransparent substance, when turned in such a manner on its 

 axis as partly to reflect and partly to transmit the rays of light, (fur 

 it will never decompose if turned at right angles to the sun,) con- 

 denses and reflects fringes of blue, red, and j-ellow, from each of 

 its angles; and these fringes of reflected light, being carried forward 

 through the prismatic planes by the incident and undecomposed 

 light, intermix by their different refrangibilities, and form a spec- 

 trum of seven colours; and as there are three angles in every 

 prism, so there are two spectra always formed, in the same manner 

 as three stri]>s of paper pasted parallel to one another on the window 

 will also form two spectra. As I am well aware that my experi- 

 ment and opinions are in opposition to authorities of the first 

 respectability in science, and as I am also certain that science and 

 liberality always go hand in hand, I rest my idea on experimental 

 inquiry. To show that the decomposition of light takes place only 

 at the prismatic angles, and arises entirely from those fringes of 

 reflected light, 1 made the following experiment. I placed my 

 prism on a table at an open window, through which the sun was 



