SIR DAVID BREWSTER ON THE ANATOMICAL AND OPTICAL 
:<8 
these lines are not now visible in the indurated lens, I cannot even form a conjecture 
respecting their origin. 
4. Stellio Gecko and Frog. — The lenses of these animals require to be re-examined. 
5. Salmon. — The length of the septa in the lens of the salmon is less than those in 
the hare and rabbit, and the teeth upon the fibres are extremely distinct. The lens 
depolarizes three series of luminous sectors, the inner and outer series being negative 
and the intermediate series positive *. The polarizing structure of the cornea was 
negative, and it depolarized very high tints at its junction with the sclerotic coat. 
The structure of the sclerotic coat is very remarkable. In the eye which I examined, 
the thickness of the sclerotic was about the fifteenth of an inch, and with a sharp 
knife it could be cut like a piece of cheese. It had a milky transparency like some 
opals. When a slice with parallel faces, nearly perpendicular to the surface, was ex- 
posed to polarized light, it exhibited the system of biaxal rectilinear fringes exactly 
like those in a plate of glass heated by boiling water or oil, and in the act of rapid 
cooling. The same structure exists in the sclerotic coat of the Cod. - 
6. Dolphin. — The lens of the dolphin is decidedly an oblate spheroid, the axis of 
which is that of vision. In an indurated lens the axis of the spheroid is 0 - 254 of an 
inch, and the equatorial diameter 0‘307. The teeth of the fibres are small and irre- 
gular, like those of quadrupeds. 
7. Shark. — The lenses of the common and blue-eyed shark have the same structure. 
The sclerotic coat has the remarkable property (when cut in the manner already de- 
scribed in the case of the salmon) of depolarizing light like a plate of bent glass ^ ; 
but, what is very curious, the concave side of the sclerotic has the same action upon 
light as the convex side of the bent glass. 
8. Alligator. — The lens of the alligator is nearly spherical : the teeth of the fibres, 
as in the dolphin, are shorter than those in fishes. 
9. Skate. — The lens of this fish depolarizes three series of luminous sectors, but 
the inner series is not so distinct and near the axis as those of the cod. The inner 
and outer series are negative, and the intermediate series positive. The horny scle- 
rotic of the skate has the very same polarizing structure as that of the salmon. The 
teeth of the fibres of the lens are exceedingly small. 
10. Thornback. — The fibres are very delicate, and their teeth small but distinct. 
The lens depolarizes luminous sectors ; but from the imperfection of the lens I could 
not observe their character. 
I 1 . Boneto. — The lens is an oblate spheroid, whose axis is that of vision. The 
teeth of the fibres are very distinct §. 
12. Sword-fish. — The diameter of the lens of this fish which I examined was IT 0th 
of an inch. The teeth of the fibres were so distinct that three circles of the secondary 
colours produced by them were distinctly seen||. 
Philosophical Transactions, 1816. 
§ Ibid., 1833, p. 332. 
t Ibid., 1816. 
|| Ibid., 1833, p. 327. 
J Ibid., 1816. 
