Qgo Dr. Brewster on the multiplication of images , and the 
prisms of calcareous spar separated by a film of sulphate of 
lime so placed that each pair of the four images is tinged 
with the complementary colours as described in Sect. IV. A 
lens L is cemented either upon the anterior or posterior sur- 
face of the compound prism, or may be kept separate from 
the prism at L, but whatever be its position, it must always 
enable the eye at E to see the aperture with perfect distinct- 
ness, and the focal length of the lens must be so adapted to 
the magnitude of the aperture, that the images of it can be 
sufficiently separated by the universal motion of the ball CD. 
The interior of the tube being covered with a black pigment, 
the instrument is ready for use. If we direct it to the sky, or 
to any luminons object, four brilliantly coloured images of 
the aperture will be distinctly seen, the colour of the two 
middle images being complementary to that of the two ex- 
treme images. By moving the ball in the socket, the colours 
will constantly change, and the images will sometimes over 
lap, and sometimes separate, exhibiting the finest variety of 
hues, and pleasing the eye by their combinations, and by the 
soft harmony of their contrasts.* 
In the instrument where it is necessary to polarise the 
light by black glass, or japanned metal, there is no less than 
i|-ths of the incident pencil lost by reflection, while in the 
preceding instrument the light lost by transmission is very 
small. From this cause, the brightness of the colours is 
incomparably greater, and they may even be distinctly seen 
in candle light, by directing the aperture to a piece of white 
paper held near the candle. 
* The phenomena will admit of many beautiful variations, by using several films of 
sulphate of lime, having their axes variously inclined to one another. 
