1814.] Experiments on Light. 277 



colourless ; but when we look to the frames we perceive an artificial 

 rainbow, of reflected blue, red, and yellow. Any opaque sub- 

 stance, as a piece of black cloth or paper, when pasted on the 

 window, will produce the same eflfect ; and the more dense or dark, 

 the deeper the tints or fringe. The north or top of the paper will 

 be fringed with blue, the south or bottom with red and yellow rays. 

 Now it is evident, if light is decomposed by merely passing 

 through the prism according to the different refrangibilities of its 

 coloured rays, that light admitted through the panes should be 

 equally decomposed with that in the vicinity- of the opaque frames. 

 To place this argument in a stronger point of view, I made the 

 following experiment. I cut two holes in my window shutter, one 

 the diameter of i of an inch, mentioned by Sir Isaac Newton, the 

 other the diameter of four inches ; and having darkened the room, 

 and applied a prism, I found that the small aperture admitted light 

 tinged with the seven colours, which I could receive on a sheet of 

 white paper ; the larger orifice was also fringed round with seven 

 prismatic colours, and pencils of white light passed through the 

 centre. Here I must again ask, if white incident light were decom- 

 posed by merely passing through the prism, why was not that 

 coming through the centre equally decomposed with that at the 

 edges. And however contrary to received opinion, I am confident 

 it is nevertheless true that incident light has never yet been decom- 

 posed, but that all experiments hitherto made have been on light 

 condensed and reflected by opaque substances. If we paste a piece 

 of black cloth on the window, whose colour, as 1 have shown in 

 my last communication on blackness, (see Philosophical Magazine,) 

 arises from the reflection of condensed rays of blue, red, and 

 yellow, on applying the prism a fringe of red and yellow appears 

 at the south : this does not proceed from a decomposition of inci- 

 dent light striking on the edges of the cloth, but it proceeds from 

 an actual decomposition of the condensed coloured rays of the 

 black cloth itself. The prism decomposes these three primary 

 colours according to the order of their different refrangibilities; and 

 '*& the red and yellow rays are more refrangible than the blue, as I 

 shall show in my next communication, they are brought down by 

 the prism, and tiic black cloth remains of a blue colour : the far- 

 ther we move from the window, the more refrangible the red and 

 yellow rays become, and consequently the decomposition is the 

 greater. In this experiment the nortii of the clolh reflects blue 

 rays, the south red and yellow, proving in the most satisfactory 

 manner that there are but three primary coloui-s; and as all the 

 secondary or mixed colours can he formed by blue, red, and yellow, 

 to call oihers into existence would be contrary to the beautiful 

 simplicity of nature, and unnecessary. But it might be asked, if 

 there arc but tiiroe primary colours, how did Sir Isaac Newton 

 produce a spt-ctrum of seven ? The following experiment will 

 explain this. Faste i^ strip of black cloth or paper six inches by three 

 «u the window, on the south you perceive a fringe of reflected red 



