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of the lens occupy positions which are reversed with respect to one 

 another ; thus in the simple case of the double septa at each pole, 

 the line formed by those of the posterior surface is situated at right 

 angles to that formed by the septa of the anterior surface. Where 

 there are three divergent septa at each pole, the direction of those 

 on the one side bisect the angles formed by those on the other side ; 

 and again, where the septa form a rectangular cross, those of one 

 surface are inclined 45 : to those of the other surface. 



It follows as a consequence of this configuration of the series of 

 points which constitute the origins and terminations of the fibres, 

 that all the fibres, with the exception only of those proceeding in a 

 direct line from the extremities of any of the septa, must, in their 

 passage from the one surface to the other, follow a course more or 

 less contorted ; and must form lines of double curvature ; that is, 

 curves of which none of the portions lie in the same plane. 



The fibres of the lenses of quadrupeds gradually diminish in size 

 from the equator or margin of the lens, where they are largest, to 

 their terminations in the anterior or posterior septa. They are 

 united together by small teeth like those of fishes : but. generally 

 speaking, the teeth are smaller and less distinctly pronounced, and 

 sometimes they are not seen without great difficulty. 



In the lens of the turtle, as well as in that of several fishes, the 

 arrangement of the fibres, instead of being symmetrical on the two 

 sides, as is the case in all the preceding instances, is different on the 

 anterior and posterior surfaces: there being two septa on the for- 

 mer, but none in the latter, which presents only a single polar point 

 of convergence. 



The author has directed much of his attention to the optical pro- 

 perties of these structures, The lens of the salmon depolarizes three 

 series of luminous sectors ; the inner and outer series being negative, 

 and the intermediate series positive. The polarizing structure of 

 the cornea is negative, and it depolarizes very high tints at its junc- 

 tion with the sclerotic coat. When a slice cut from the sclerotica 

 nearly perpendicularly to the surfaces, and with parallel faces, is 

 exposed to polarized light, it exhibits the system of biaxal rectilineal 

 fringes, exactly like those in a plate of glass heated by boiling wa- 

 ter or oil, when in the act of rapid cooling. The same alternation 

 of properties with regard to polarization in the successive strata of 

 the substance of the crystalline lenses is exhibited by other fishes 

 which the author examined. 



With respect to the final cause of these highly complicated ar- 

 rangements, it is reasonable to conceive that the gradually increas- 

 ing density of the fibres in each successive stratum from the surface 

 to the centre is intended to correct spherical aberration : but the 

 design of the other properties resulting from the arrangement of the 

 fibres with reference to septa, in all their variations of number and 

 position, and more especially the alternations of positive and nega- 

 tive structures, as exhibited by the action of the different strata in 

 polarized light, has not even excited the ingenuity of conjecture, 

 and will probably remain among the numerous problems destined to 

 exercise the sagacity of another age. 



