194 Incident or impingent Light not decomposable 
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 were 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 de- 
_ composed with that in the vicinity of the opaque frames. To 
place this objection in a stronger point of view, I made the fol- 
lowing experiment. 
I cut two holes in my window-shutter, one the diameter of a 
quarter of an inch, mentioned by Sir Isaac Newton, the other the 
diameter of four inches ; and having darkened the room, and ap- 
plied a prism, | 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 pris- 
matic colours, and pencils of white light passed through the centre. 
Here I must again observe, if white incident light were de- 
composed 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 con- 
fident it is nevertheless true, that incident light has never yet 
been decomposed, but that all experiments hitherto made have 
been on light condensed and reflected by opaque substances. Ifwe 
paste a piece of black cloth on the window, whose colour, as I 
have shown in my last communication on blackness, 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 incident 
light striking on the edges of the cloth, but it proceeds from an 
actual decomposition of the coudensed coloured rays of the black 
cloth itself. ‘The prism decomposes these three primary colours 
according to the order of their different refrangibilities; and as 
' the red and yellow rays are more vefrangible than the blue, as I 
shall. show in my next communication, they are brought down 
by the prism, and the black cloth remains ofa blue colour. The 
further 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 north of the cloth reflects 
blue rays, the south red and yellow, proving in the most satis- 
factory manner that there are but three primary colours; and as 
all the secondary or mixed colours ean be formed of blue, red, _ 
and yellow, to call others into existence would be contrary to the 
beautiful simplicity of nature, and unnecessary. But it might be 
asked, If there are but three primary colours, how did Sir Isaac 
Newton produce a spectrum of seven ? 
The following experiment will explain; Paste a strip of black 
cloth or paper, six inches by three, on the window ; on the south 
you 
