42 Dr. Brewster's experiments on the 
m, all of which are much inferior in brilliancy to those with 
which n is encircled. 
When equal parts of rosin and oil of mace are mixed toge- 
ther, a film formed out of the mixture depolarises, imperfectly, 
the four luminous sectors. 
53. Tallow. When tallow is melted between two plates of 
glass, and then slowly cooled, it exhibits no optical indica- 
tions of a crystallized structure. After having stood live or 
six days, an incipient crystallization is exhibited in the pro- 
perty which it acquires of depolarising a small portion of 
nebulous light. This nebulosity gradually increases : about 
the eleventh day, it assumes an imperfect resemblance to the 
four luminous sectors produced by oil of mace ; and about 
the sixteenth day, the form of the sectors is fully developed. 
Another plate formed of the fat of mutton, after standing five 
months, has acquired in some parts the property of depolaris- 
ing a portion of nebulous light, while, in other parts, it de- 
polarises a small part of the bright image. In a third plate, 
suddenly cooled by immersion in cold water, very faint traces 
of four large luminous sectors were visible by a careful ex- 
clusion of extraneous light. 
54. Tortoise shell depolarises light in every position, and 
produces the coloured rings by polarised light. When a 
candle is viewed through tortoise shell, it is surrounded with 
a double halo, which becomes elliptical by inclining the plate 
to the incident rays. When the polarised light of a candle is 
depolarised, the restored image is surrounded with four very 
iaint luminous sectors, like those in oil of mace. 
55. Heated glass. When glass is brought nearly to a red 
heat, it depolarises light in every position, and the quantity 
