30 



From these observations, and from the peculiar provision of fat 

 laid up by the tadpole previous to its metamorphosis into the frog, 

 Sir Everard Home is led to consider a certain portion of oil as ne- 

 cessary for the formation of bone, and to observe that the proportion 

 of fat in different ova, corresponds with the greater or less degree of 

 hardness of bone that is about to be produced. 



On the Structure of the Crystalline Lens in Fishes and Quadrupeds, as 

 ascertained by its Action on Polarized Light. By David Brewster, 

 LL.D. F.R.S. Land. andEdin. In a Letter addressed to the Right 

 Hon. Sir Joseph Banks, Bart. G.C.B. P.R.S. Read June 20, 1816. 

 [Phil. Trans. 1816, p. 311.] 



The author having found that in many instances depolarization 

 depends upon variations of density in the bodies through which it is 

 transmitted, concluded that corresponding effects would be produced 

 by the crystalline lens of the eye, which is well known to increase 

 in density towards the centre. 



By immersing the crystalline lens of a cod in Canada balsam, the 

 refraction at its exterior surface was so far removed, that the effects 

 of its internal texture could be examined independent of its external 

 spherical form. Under these circumstances, when it was exposed to 

 polarized light, with its axis of vision parallel to the beam of light, 

 it exhibited three concentric circles of light separated by two dark 

 circles, and intersected by a black cross so as to be divided into twelve 

 luminous sectors. By varying the inclination of the axis, these ap- 

 pearances varied in a manner that can scarcely be described without 

 the assistance of the drawings which accompany this paper. 



By removing successively the capsule and outer portions of the 

 lens, the exterior circle of light was first obliterated ; and then the 

 second disappeared, so that ultimately there remained only the cen- 

 tral light intersected by a black cross. 



On examination of the variation of tints of colours produced by the 

 combined effect of the crystalline with a plate of sulphate of lime, it 

 appeared to the author that the central nucleus, and an exterior 

 spherical coat, are in a state of dilation, while the intermediate coats 

 are in a state of contraction. 



In the crystallines of sheep and oxen, the author observed a corre- 

 spondent texture ; but in these there appeared only one series of lu- 

 minous sectors. 



By examination of the cornea also, Dr. Brewster found a texture 

 similar to that of the nucleus of the crystalline, both in fish and qua- 

 drupeds ; but the sclerotic coat has merely a confused power of de- 

 polarization, similar to that of a mass of crushed jelly of isinglass, a 

 property which does not really belong to its whole substance, but 

 solely to a thin membrane that covers it externally. 



From these experiments Dr. Brewster infers, that all the parts of 

 the crystalline of fishes correspondent to the two dark concentric 

 circles, exercise no action upon polarized light ; that a central nu- 



