34 Dr. Brewster’s experiments on the 
by the heat of the hand, the film is almost perfectly transpa- 
rent by transmitted light, though of a milky white appearance 
by reflected light. It has not the property of depolarisation 
when the polarised pencil is incident vertically , but it pos- 
sesses it in a very perfect manner at an oblique incidence , and 
exhibits the segments of coloured rings. 
6. Cells of the bee. The waxen partitions of the honey-comb, 
when rendered transparent by layers of Canada balsam, de- 
polarise light in every position. 
7. Manna. This substance, when melted by heat between 
plates of glass, depolarises light in every position. 
8. Camphor. A small piece of this substance when pressed 
between two plates of glass, without the application of heat, 
depolarises light in every position. 
9. Balsam of Tolu. When a thin plate of this substance is 
formed between plates of glass with the assistance of a gentle 
heat, it depolarises light in every position ; and a considerable 
degree of heat is necessary to deprive it of its crystalline 
structure. When it is allowed to cool very slowly, it does 
not acquire the property of depolarisation. 
10. Withered film at the root of the Calla Ethiopica. This ve- 
getable film depolarises light, and possesses distinct neutral 
axes which are parallel and perpendicular to the stalk of the 
plant, or to the parallel veins in the film. 
11. The fibres of flax, hemp, and cotton . These vegetable 
fibres depolarise light, and have perfect neutral axes parallel 
and perpendicular to the axes of the fibres. 
1 2. The thin white semi-transparent leaf of the sea-weed depo- 
larises a small portion of light, and its neutral axes are parallel 
and perpendicular to the axis of the leaf. 
