the Cause of coloured concentric Rings . 231 
upon the pasteboard, in a quantity sufficient to make them 
“ visible.”* 
Mr. Brougham, after having considered what the two au- 
thors I have mentioned had done, says, “ that upon the whole 
“ there appears every reason to believe that the rings are 
formed by the first surface out of the light which , after re- 
“ flection from the second surface, is scattered, and passes on 
“ to the chart.* ’-f* 
My own experiment is as follows. I placed a highly polished 
7 feet mirror, but of metal instead of glass, that I might not 
have two surfaces, at the distance of 14 feet from a white 
screen, and through a hole in the middle of it one-tenth of an 
inch in diameter I admitted a beam of the sun into my dark 
room, directed so as to fall perpendicularly on the mirror. In 
this arrangement the whole screen remained perfectly free 
from light, because the focus of all the rays which came to the 
mirror was by reflection thrown back into the hole through 
which they entered. When all was duly prepared, I made 
an assistant strew some hair-powder with a puff into the beam 
of light, while I kept my attention fixed upon the screen. As 
soon as the hair-powder reached the beam of light the screen 
was suddenly covered with the most beautiful arrangement of 
concentric circles displaying all the brilliant colours of the rain- 
bow. A great variety in the size of the rings was obtained by 
making the assistant strew the powder into the beam at a 
greater distance from the mirror; for the rings contract by an 
increase of the distance, and dilate on a nearer approach of the 
powder. 
* Priestley’s History, &c. on the‘Colours of thin Plates, p. 515. 
•j- Phil. Trans, for 1796, p. 216. 
H h 2 
