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 occa- 

 sioned ; 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 mem- 

 brane 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. 



On the Communication of the Structure of doubly -refracting Crystals 

 to Glass, Muriate of Soda, Fluor Spar, and other Substances, by 

 mechanical Compression and Dilatation. By David Brewster, LL.D. 

 F.R.S. Lond. and Edin. In a Letter addressed to the Right Hon. 

 Sir Joseph Banks, Bart. G.C.B. P.R.S. Read February 29, 1816. 

 [Phil. Trans. 1816,;?. 156.] 



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 me- 

 chanical. 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 in- 

 creased. 



When two plates under a state of compression are combined trans- 

 versely, 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 thick- 



