VISION. 3 



where copiously exemplified. On the general phy- 

 siological principle, also, that the more stimulus an 

 organ can bear the less sensible it must be, we may 

 conclude that the two circumstances of piercing sight 

 and capability to gaze on the sun are incompatible. 

 Accordingly we find that animals which prey in the 

 night, and white animals with red eyes (albinos), 

 while they can see with a faint glimmer of light on 

 account of their extreme sensibility of vision, are so 

 overpowered by the glare of the sun's light that they 

 can scarcely see at all. 



There is indeed a peculiarity in the eyes not only of 

 the eagle but of all birds, tending to defend them from 

 too strong light, to which it may be interesting to 

 advert. In the eye of man and many other animals, 

 including birds, a beautiful provision is made for sup- 

 plying it with the moisture which is indispensable to 

 render its restless motions easy. For this purpose a 

 fountain or gland, situated in man within the upper 

 and outer part of the bony socket, affords a constant 

 supply of tears, about three or four ounces of which it 

 has been computed are discharged upon the human eye- 

 ball every twenty-four hours. Besides this there is an 

 ointment prepared in the beautiful little glands, about 

 thirty in number, interspersed within the fine skin of 

 the inner eyelid near to the roots of the eyelashes, 

 appearing when magnified like studs of minute pearls. 

 This ointment, which, according to M. Majendie, is 

 of a glairy consistence, like white of egg, is dissolved 

 and diluted by the tears, and the whole is constantly 

 spread over the eyeball by the sweep of the eyelids, 

 which act like valves, and are composed of semi- 

 transparent muscular substance, attached to a ring 

 of gristle or cartilage, which is hinged on the adjacent 

 bone, and gives the eyelids firmness and preserves their 

 shape. Our eyes are besides elegantly fringed with 



