198 



EYE. 



of the sphere, again form another axis or centre, 

 and return in the direction O F, and the fila- 

 ments O F, again on the other side of the 

 sphere, collect round a third centre, and thence 

 return in the direction KM; so that the fila- 

 ments which are on this side of the sphere 

 collect round a third centre, and thence return 

 in the direction K M; so that the filaments 

 which are on this side the shortest, on the other 

 side are the longest, and those which there are 

 the shortest are here the longest." Annexed is 

 Leeuwenhoek's representation (Jig. 119). 



Fig. 119. 



Sir David Brewster says that the direction of 

 the fibres is different in different animals; the 

 simplest arrangement being that of birds, and 

 the cod, haddock, and several other fishes. In 

 it the fibres, like the meridians of a globe, con- 

 verge to two opposite points of a spheroidal or 

 lenticular solid, as in the annexed figure. 



Kg. 121. 



Dr. Young differs from Leeuwenhoek as to 

 the arrangement of the fibres and other parti- 

 culars, and in his last paper corrects the de- 

 scription given by himself in a former one ; he 

 says, " The number of radiations (of the fibres) 

 is of little consequence , but I find that in the 

 human crystalline there are ten on each side, 

 not three, as I once from a hasty observation 

 concluded." " In quadrupeds the fibres at 

 their angular meeting are certainly not conti- 

 nued as Leeuwenhoek imagined." Beneath is 

 Dr. Young's last view of the arrangement of 

 the fibres, which Dr. Brewster has shown to be 

 incorrect, but the introduction of which is jus- 

 tified by the source from which it is derived. 



Fig. 120. 



The second or next simplest structure he 

 detected in the salmon, shark, trout, and other 

 fishes ; as well as in the hare, rabbit, and por- 

 poise among the mammalia; and in the alli- 

 gator, gecko, and others among reptiles. Such 

 lenses have two septa at each pole, as in the 

 annexed figure. 



Fig. 122. 



The third or more complex structure exists 

 in mammalia in general, " in which three septa 

 diverge from each pole of the lens, at angles of 

 1 20, the septa of the posterior surface bisect- 

 ing the angles formed by the septa of the ante- 

 rior surface, as in the annexed figure (fig. 12 3). 



