Page:Proceedings of the Royal Society of London Vol 2.djvu/71

Rh its margin is surrounded by a broad loose membrane, capable of very close application to any surface on which it is placed.

By elevation of the plates, a degree of exhaustion is thus occasioned; and the fish thereby firmly attaches himself to the shark, or to any other object.

In the same manner it would appear that the transverse serratures of the bottom of the toes of the lizard, by their elevation, occasion a degree of exhaustion or partial vacuum, confined by the broad membrane which is attached all round each of the toes.

The author is of opinion, that the feet of the common fly act upon the same principle. Their under surfaces, when highly magnified, appear to be concave, as they are represented by Kellar; and he thinks it cannot be doubted that these cavities are employed to rarify the air between them and the surfaces to which they are applied, and thus support the weight of the fly, in opposition to gravity, when suspended from a ceiling. 



The subjects here chosen for experiment are such bodies as have in general no power of polarizing or depolarizing light, and the means employed for communicating these properties are purely mechanical. In the first instance, a piece of plate-glass was taken, and compressed edge-wise between two screws, and was found to polarize light in every part of its breadth, with depolarizing axes, making an angle of 45 with the edges of the plate.

When a narrow slip of plate-glass is attempted to be bent edge-wise, the inner edge becomes compressed sufficiently to produce the effect of depolarization; and the exterior edge of the curve, by being dilated, also depolarizes: but the characters of the fringes of colour produced in the two cases are different; since those which arise from compression are such as are produced by calcareous spar and beryl; but those caused by dilatation of the exterior edge are such as appear from the action of sulphate of lime, quartz, and other bodies of that class.

The author observes, that the tints polarized ascend in Newton's scale, in proportion as the forces of compression or dilatation are increased.

When two plates under a state of compression are combined transversely, the same phenomena are exhibited as by means of a plate formed of a doubly-refracting crystal.

The effect of two plated of compressed glass, similarly placed, is the same as that of a double plate; but if they be placed transversely, then the tints are such as are due to the difference of their