242 Mr. Brougham's Experiments and Observations on 
tests I could think of, it was white inclining to yellow, and 
of the same nature and constitution with the sun’s direct light ; 
but if any ray was stopped before coming to the lens, the focus 
was a mixture of the remaining rays ; and the chart being 
moved a little farther round, the image was formed on it, the 
colours being in an inverted order. At the focus I held a re- 
flector, and there were formed images of all the seven colours, 
as in the sun’s direct light (Exp. 1.); if the light was suffi- 
ciently strong, and the desk near the window-shut hole, one 
of these could even be collected by a second lens into a white 
focus. This experiment is rendered more uniform by substi- 
tuting for the lens a concave metallic mirror, and placing at 
the focus another mirror to reduce the rays into a beam, which 
may be made of any composition we please, by stopping one 
or more of the colours at the hole in the desk. I observed in 
the course of these experiments a phaenomenon worth men- 
tioning ; if a comb (as in Newton’s experiment*) be very 
swiftly moved before one of the images, or more, a sensation 
of white is produced ; but this is still more evident, if the pin 
be swiftly moved round its axis, for then the images move 
also, and running into one another, cause a sensation of per- 
fect whiteness. 
Obs. 7. I let an image through the hole in the desk, and 
viewed it through a glass prism, holding its axis parallel to the 
sides of the image, and its refracting angle upwards ; I found 
that, if the image was bright and free from white light, the 
colours were not changed by the refraction ; but, if it was 
mixed and diluted with white, the prism, decompounding the 
white, caused the image to appear violet at one side, and red 
* Optics, Book I. Part II. Prop. 5. 
