THE EYE AND THE PHYSIOLOGY OF VISION. 553 



when the embryonic development of the eye is studied. 

 Very early in such development the retina can be seen folded 

 in such a way that what would naturally have been towards 

 the light is turned away from the light. In the invertebrates 

 the rods and cones turn towards the light, and eyes having 

 such an arrangement are, therefore, usually referred to as 

 invertebrate eyes. 



THE EYE AS A PURELY PHYSICAL INSTRUMENT. 



The ability of the eye to bend rays of light passing 

 through its refracting media and to produce an image on or 

 near the retina is a purely physical property. Such results 

 would occur in an eye that had been removed from the 

 body. A stimulation of the rods and cones, however, and 

 the consequent production of nervous impulses, which, when 

 reaching the brain are interpreted as color, are physiologi- 

 cal processes and can be duplicated in no artificially con- 

 structed instrument. Before proceeding to the physiology 

 of the eye it seems desirable to call attention to those ar- 

 rangements and properties which it shares with all optical 

 instruments, and to the defects which may arise in the eye 

 in common with other optical instruments. 



THE NORMAL OR EMMETROPIC EYE. 



When the eyeball and its refracting media are so con- 

 structed that parallel rays of light shall meet in a focus on 

 the retina when the eye is in a resting condition, it is called 

 a normal or emmetropic eye. Rays of light are for prac- 

 tical purposes considered as parallel when they come from 

 a point removed from the eye about twenty-five feet or more. 

 An emmetropic eye, therefore, may be defined as one which 

 in its natural resting position is in focus for distant objects. 

 Many eyes do not conform to this requirement, but show in 

 a greater or less degree one or more of the following optical 

 defects: 



1. Myopia, or Short Sight. In a myopic or short- 

 sighted eye the image falls in front of the retina, produced 



