COMPARATIVE PHYSIOLOGY OF INSECTS. 183 



He found that the hard central portion is composed of 

 a succession of concentric, and perfectly transparent, 

 spheroidal laminae, 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 

 towards the two poles of the spheroid. 



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 ta- 

 per at each end, as they approach 

 the points of convergence. The 

 breadth of the fibres in the most ex- 

 ternal layer, at the equator, is about 

 the 5,500th part of an inch. 



Having his curiosity excited by 

 this singular structure, Sir David Brewster continued 

 his miscroscopic 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 



JWliMlUmliMl'.M'.'.W',. W Ni I* 1 fflffl M T '',.'] . * 1 r*I 1/ IA! 



m 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 amounts to the number of 



62,500,000,000. 



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 sizes. 



"What is said of the eyes of birds 1 



