COMPARATIVE PHYSIOLOGY OF VISION. 179 



550. He found that the hard central portion is composed 

 of a succession of concentric, and perfectly transparent, 

 spheroidal lamina?, the surfaces of which, though appa- 

 rently smooth, have the same kind of irridescence as the 

 shell called mother-of-pearl, and arising from the same 

 cause, namely, the occurrence of regularly-arranged 

 lines, forming a striated surface, but so fine as to be 

 detected only by a powerful magnifier. These lines, which 

 mark the edges of the separate fibres, composing each 

 lamina, converge like meridians from the equator toward 

 the two poles of the spheriod. 



551. This appearance, magnified, is shown by Fig. 110, 

 where it will be observed that these fine lines converge 

 to a centre at the upper part of the 



figure. The fibres themselves are not 

 cylindrical, but flat ; and they taper 

 at each end, as they approach the 

 points of convergence. The breadth 

 of the fibres in the most external lay- 

 er at the equator, is about the 5,500th 

 part of an inch. 



552. Having his curiosity excited by 



this singular structure, Sir. David Brewster continued 



his microscopic observations on the same substance, 



and by using a very high power, he further discovered 



that these fibres are locked together at the edges by a 



series of teeth resembling those 



of rack-work, as represented by 



Fig. 111. He counted the teeth 



in a single fibre, and found that =^ 



they amounted to 12,500, and as 



he ascertained that the whole 



lens contained about 5,000,000 



of fibres, the whole number of 



these teeth in a single lens amount to the number of 



62,500,000,000. 



553. Structure of the Eyes of Birds. The eyes of birds 

 are very large when compared with the head, or with those 

 of the other animals of the same size. 



What is said of the eyes of birds ? 



