Page:Proceedings of the Royal Society of London Vol 2.djvu/82

Rh 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 necessary 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.

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 appearances 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 central 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 correspondent texture; but in these there appeared only one series of luminous 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 quadrupeds; but the sclerotic coat has merely a confused power of depolarization, 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