40 Dr. Brewster’s experiments on the 
52. Oil of mace. This soft solid exhibits optical properties 
of a very peculiar character. When it is pressed into a thin 
film between two plates of glass without the aid of heat, it 
depolarises light in every position. It melts nearly at the 
temperature of blood heat, and takes a long time to cool and 
crystallize. If a thin plate of it is melted, and afterwards 
cooled between two pieces of glass, the image of a candle 
when seen through some parts of it, is encircled with a halo 
of nebulous light, varying in different plates from o° to 16 0 in 
diameter, and having its central parts of a bluish colour, and 
the circumference of a reddish brown hue. In other parts of 
the film the halo disappears. 
When the polarised light of a candle is transmitted through 
those parts of the film which do not produce the halo, it is 
depolarised in every position ; but when the light is trans- 
mitted through the marginal parts, or those which produce 
the halo, the oil of mace restores four wings of light or four 
luminous sectors , in the centre of which is the place of the eva- 
nescent image. Through intermediate parts of the film, it 
depolarises two luminous images of the candle, separated by 
a narrow dark space, and manifestly formed of condensed 
nebulous light. Upon moving the film from the position 
which gives the luminous sectors, into that which gives the 
complete image of the candle, the wings or sectors gradually 
diverge from their common centre, and then vanish ; and 
upon moving the film from the same position into that which 
gives the two luminous images, each adjacent pair of the 
sectors approach one another, and are condensed into two 
luminous semi> circles, which form the two images already 
mentioned. In the dark space between these two images, the 
