OF SIGHT. 173 



forehead, and in some measure screen them from too 

 strong' a light. 



2b'8. To lubricate the eyes, to preserve their bright- 

 ness, and to wash away foreign matters, is the office of 

 the tears. Their chief source is a conglomerate gland 

 placed in the upper and exterior part of the orbit. It 

 has numerous but very fine excretory ducts, which are 

 said to discharge about two ounces of tears upon each 

 eye during the twenty-four hours : the tears are after- 

 wards absorbed by the puncta lachrymalia, the function 

 of which may, in a certain sense, be compared to that of 

 the lacteals in the villous coat of the small intestines ; 

 from the puncta they are conveyed through the snails' 

 horns, as they are called, into the lachrymal sac, and 

 thence pass into the lower meatus of the nostrils.* (A) 



2(>9. Thus much it was necessary to premise upon 

 the structure of the organ of vision. We now come to 

 the function of the organ, to the explanation of vision. 



Rays of light falling upon the cornea at an angle 

 more acute than forty-eight degrees, pass through it, 

 and, from both its density and figure, are considerably 

 refracted towards the axis of the eye. On entering the 

 aqueous humour they experience rather a less degree 

 of refraction. 



Those rays which penetrate the pupil and are received 

 by the lens, are still more refracted on account of the 

 greater density of this medium. 



The less density of the vitreous humour prevents the 

 focus of rays from being too small, but allows it to 

 fall elongated upon the retina and exhibit the image 



* J. Chr. Rosenmiiller, Organa Lachrymalium Partiumque E.rtemarum 

 Omli Hitmani Descriptin Anutamica. Lips. 1797. 4to. 



