THE EYE AS AN OPTICAL INSTRUMENT. 207 



necessary to devise trustworthy raethods in order to 

 ascertain the precise limits of the power of accommoda- 

 tion even with inexperienced and uninstructed patients. 

 It became apparent that very different conditions had 

 been confounded as short sight and long sight, and this 

 confusion bad made the choice of suitable glasses un- 

 certain. It was also discovered that some of the most 

 obstinate and obscure aflfections of the sight, formerly 

 reputed to be 'nervous,' simply depended on certain 

 defects of accommodation, and could be readily removed 

 by using suitable glasses. Moreover Donders * proved 

 that the same defects of accommodation are the most 

 frequent cause of squinting, and Von Grraefe* had already 

 shown that neglected and progressive shortsightedness 

 tends to produce the most dangerous expansion and 

 deformity of the back of the globe of the eye. 



Thus connections were discovered, where least expected, 

 between the optical discovery and important diseases, 

 and the result was no less beneficial to the patient than 

 interesting to the physiologist. 



We must now speak of the curtain which receives the 

 optical image when brought to a focus in the eye. This 

 is the retina, a thin membranous expansion of the optic 

 nerve which forms the innermost of the coats of the eye. 

 The optic nerve (Fig. 2, 0) is a cylindrical cord which 

 contains a multitude of minute fibres protected by a 

 strong tendinous sheath. The nerve enters the apple of 

 the eye from behind, rather to the inner (nasal) side of 

 the middle of its posterior hemisphere. Its fibres then 

 spread out in all directions over the front of the retina. 

 They end by becoming connected, first, with ganglion cells 

 and nuclei, like those found in the brain ; and, secondly, 



• Professor of Physiology in the University of Utrecht. 



' This great ophthahnic surgeon died in Berlin at the early age of forty-two. 



