THE EYE AS AN OPTICAL INSTRUMENT. 183 



takes a certain position, and then all he sees is a faint misty re- 

 flection. But this faint reflection was destined to become a 

 shining light in a dark corner of science. It was in fact the 

 first appearance observed in the living eye which came directly 

 from the lens. Sanson immediately applied his discovery to 

 ascertain whether the lens was in its place in cases of impaired 

 vision. Max Langenbeck made the next step by observing that 

 the reflections from the lens alter during accommodation. These 

 alterations were employed by Cramer of Utrecht, and also inde- 

 pendently by the present writer, to arrive at an exact knowledge 

 of all the changes which the lens undergoes during the process 

 of accommodation. I succeeded in applying to the moveable eye 

 in a modified form the principle of the heliometer, an instrument 

 by which astronomers are able so accurately to measure small 

 distances between stars in spite of their constant apparent 

 motion in the heavens, that they can thus sound the depths of 

 the region of the fixed stars. An instrument constructed for 

 the purpose, the ophtltalmometer, enables us to measure in the 

 living eye the curvature of the cornea, and of the two surfaces 

 of the lens, the distance of these from each other, <kc., with 

 greater precision than could before be done even after death. 

 By this means we can ascertain the entire range of the changes 

 of the optical apparatus of the eye so far as it affects accom- 

 modation. 



The physiological problem was therefore solved. Oculists, 

 and especially Bonders, next investigated the individual defects 

 of accommodation which give rise to the conditions known as 

 long sight and short sight. It was necessary to devise trust- 

 worthy methods in order to ascertain the precise limits of the 

 power of accommodation even with inexperienced and unin- 

 structed patients. It became apparent that very different con- 

 ditions had been confounded as short sight and long sight, and 

 this confusion had made the choice of suitable glasses uncertain. 

 It was also discovered that some of the most obstinate and 

 obscure affections 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, 



