PROTECTION OF THE EYE. 311 



surrounding coloured bodies, not by the light re- 

 flected from itself. When the eye of an animal 

 is destined for the bright light of day, a black pig- 

 ment is behind the nerve, and the nerve itself be- 

 ing transparent, the ray is transmitted and lost. 

 But if it be required that the eye shall be suited 

 to the habits of an animal that prow^ls by night, 

 then there is combined with the large eye and the 

 very dilatable pupil, calculated to receive a great 

 pencil of rays, a property of reflection in the tape- 

 turn or carpet, that is, the surface at the bottom 

 of the eye on which the nerve is expanded. In- 

 stead of the black and absorbing pigment, there 

 is a secretion furnished by that surface, which, 

 like a dye, throws off* or reflects the light, or re- 

 flects it back like the silver on the back of a mir- 

 ror. This gives a second impulse to the nerve, 

 and has the eflfect of doubling the force of the im- 

 pression. 



Let us now see how an organ of the extreme 

 transparency and delicacy of the eye, is guarded 

 from injuries of another kind. 



And, first, we may observe the combination of 

 the living properties with the motion and mecha- 

 nism of the eyeball; how the extreme delicacy 

 of the surfaces of the eye has adapted to it the fine 

 sensibility seated in the eyelids and roots of the 

 eyelashes. The pain excited by the smallest par- 

 ticle that floats in the atmosphere, would be the 

 source of constant suffering, were there not con- 

 nected with and animated by the same sensibility 



