[ 446 ] 

 LXXIL Proceedings of Learned Societies, 



ROYAL SOCIETY. 



May 9. — A PAPER was read, entitled, " On the Anatomical and 

 £% Optical Structure of the Crystalline Lenses of Animals, 

 particularly that of the Cod." By Sir David Brewster, K.H., LL.D., 

 F.R.S. V.P.R.S.Ed. 



The author was led, by the observations he had made of some very 

 singular phenomena in the crystalline lenses of fishes and quadrupeds 

 when exposed to polarized light, to examine their minute anatomical 

 structure, with the view of ascertaining if it had any relation to these 

 optical appearances. He found that the crystalline lens of a cod has 

 the form of a prolate spheroid, of which the axis coincides with that 

 of vision. Its body is inclosed in an exceedingly thin and transparent 

 capsule, within which it floats without having any apparent connexion 

 with that capsule, and consists of a hard nucleus surrounded by softer 

 matter. The nucleus is composed of regular transparent laminae of equal 

 thickness, with perfectly smooth surfaces, presenting the iridescent 

 appearance peculiar to grooved surfaces, and exhibited by mother- 

 of-pearl. These apparent grooves have the direction of meridian lines 

 converging from the equator, where their breadth is greatest, to the 

 two poles, and indicating the boundaries of the component fibres of 

 the laminae. The author was enabled to trace the course of these 

 fibres to their termination very satisfactorily, when the fibres them- 

 selves could not be rendered visible by the best microscopes, by means 

 of the reflected prismatic images of a luminous object, produced by 

 interference. This method furnished also an accurate mode of de- 

 termining the diameter of the fibres at any point of the spheroid. The 

 uniform distribution of the light refracted through the lamina, as well 

 as the distinctness of the reflected images, prove that these fibres are 

 not cylindrical, but perfectly flat, and gradually tapering in breadth 

 from the equator to the poles of the lens. The thickness of each fibre 

 is at least five times less than its breadth, which, in the most external 

 layer of the equator, is about the 5500th part of an inch. 



The observation of another optical phenomenon apparent on look- 

 ing at a bright light through a thin lamina of the lens of a cod, namely, 

 that of two faint and broad prismatic images, situated in a line per- 

 pendicular to that which joins the common coloured images, led the 

 author to the further discovery of the mode in which the fibres are 

 united laterally to each other, so as to resist separation, and form a 

 continuous spherical surface. By viewing a well-prepared lamina with 

 a microscope of high magnifying power, he observed that the fibres 

 are united by a series of teeth, locking into one another, exactly like 

 those of rack-work. The breadth and depth of each tooth are about 

 the fifth part of the breadth of the fibre itself, and all the adjacent sur- 

 faces are in perfect optical contact. This denticulated structure exists 

 in the lenses of every fish which the author examined. In that of the 

 cod, the number of teeth in each fibre was found to be 12,500 j and 

 since the number of fibres in the whole lens is 5,000,000, the total 

 number of teeth amount to 62,500,000,000. 



