THE EYE AS AN OPTICAL INSTRUMENT. 199 



THE EYE AS AN OPTICAL INSTRUMENT. 



Br AUSTIN FLINT, M.IX, LL. D., 



PROFESSOR OF PHYSIOLOGY IN THE BELLEVUE HOSPITAL MEDICAL COLLEGE, NEW TORK ; 

 VISITING PHYSICIAN TO BELLEVUE HOSPITAL. 



I HAVE often wondered whether the statement, occasionally 

 made by physicists, that the human eye is not a perfect opti- 

 cal instrument, is an expression of human vanity or of an imper- 

 fect knowledge of the anatomy of the eye and the physiology of 

 vision ; and I have come to the conclusion that the latter is the 

 more reasonable theory. The approach to perfection in modern 

 telescopes and microscopes is wonderful indeed ; but as physi- 

 ologists have advanced the knowledge of vision, the so-called im- 

 perfections of the eye have been steadily disappearing ; and even 

 now there is much to learn. Viewed merely as an optical instru- 

 ment, an apparatus contained in a globe less than an inch in 

 diameter, in which is produced an image practically perfect in 

 form and color, which can be accurately adjusted almost instantly 

 for every distance from five inches to infinity, is movable in every 

 direction, has an area for the detection of the most minute details 

 and at the same time a sufficient appreciation of large objects, is 

 double, but the images in either eye exactly coinciding, enables us 

 to see all shades of color, estimate distance, solidity, and to some 

 extent the consistence of objects, the normal human eye may well 

 be called perfect. The more, indeed, the eye is studied in detail, 

 the more thoroughly does one appreciate its perfection as an op- 

 tical apparatus. 



Were it not for a slight projection of the cornea (the transpar- 

 ent covering in front) the eye would have nearly the form of a 

 perfect globe a small fraction less than an inch in diameter. It 

 lies in a soft bed of fat, is held jn place by little muscles and a 

 ligament which is so lubricated that its movements take place 

 with the minimum of friction. It is protected by an overhang- 

 ing bony arch and the eyelids, the eyelashes keeping away 

 dust, and the eyebrows directing away the sweat. Situated thus 

 in the orbit, the eyes may be moved to the extent of about 

 forty-five degrees ; but beyond this it is necessary to move the 

 head. 



The accuracy of vision depends primarily upon the formation 

 of a perfect image upon the retina, which is a membrane, sensi- 

 tive to light, connected with the optic nerve. That such an image 

 is actually formed has been demonstrated by an instrument, the 

 ophthalmoscope, which enables us to look into the eye and see the 

 image itself. Although the image is inverted, the brain takes no 

 cognizance of this, and every object is appreciated in its actual 



