STRUCTURE OF THE EYE. 



depend on the quantity which enters the eye through the pnpil, 

 and the sensitiveness of the membranous coating on which 

 it acts. 



Fig. 3. 



i 4 \\ 



Fig. 4. 



14. If, then, the pupils of eyes on Yenns or Mercury were 

 smaller, and those on Mars larger, in the same proportion as E 

 is smaller than Y and M (fig. 2), and larger than M', the mem- 

 branous coating having the same sensibility, the apparent 

 brightness of the solar light would be the same to all of them. 

 Or supposing the pupils of the eyes to have the same magnitude, 

 a like effect would be produced by imparting to the membranous 

 coatings different degrees of sensibility, the sensibility on Yenus 

 and Mercury being less, and on Mars greater, than those of the 

 eyes upon the Earth. 



15. In considering the powers of locomotion and strength con- 

 ferred upon animals on the earth, we find that they have certain 

 limitations ; that animals are capable of exercising these powers 

 for certain periods, varying, it is true, among individuals, but 

 still in the main comprised within certain narrow limits. We 

 find that after the lapse of certain intervals, bodily repose is 

 wanted. But besides the disposition to activity and locomotion 

 and the alternate want of rest, animals in general have also 



