412 OPHTHALMOLOGY 



facts the exact counterpart of which no one else can view. Yet he 

 beholds the same region that others see from somewhat different 

 standpoints; and the breadth of his perception is determined not by 

 busy running to and fro, rather by the height to which he has climbed 

 in his own proper domain for a viewpoint. 



The new conception of science recognizes its universal continuity; 

 and laying aside traditional boundaries, assigns to every definite, 

 important human interest, dominion over the territory which lies 

 nearest it. Such is the conception which recognizes a science of 

 ophthalmology. 



Ophthalmology centres in the function of vision; a gateway 

 perhaps the most important gateway between the objective and 

 the subjective. From this centre of its domain, highways and bypaths 

 go out in all directions, each leading to other domains of science, 

 nearer or more remote. They run for a time fairly in the domain of 

 ophthalmology, they end fairly at the centre of some other science; 

 but where they cross the border lying between, no man shall say. 

 The time devoted to this address is to be used in pointing out a few of 

 the salient features noticeable from this particular centre of know- 

 ledge; in tracing the direction toward which the paths that centre here 

 extend, and in indicating a few things of especial value that we are 

 able to offer from our cultivation of the field of ophthalmology, or 

 hope to import from other fields of activity. 



The central fact of ophthalmology is the conversion of the light 

 impulse into the nerve impulse, and this not in a single general act 

 but by a myriad of sharply differentiated actions. We receive through 

 the eye not merely a uniform impression of general external luminos- 

 ity. Through this gateway comes a message from each separate parti- 

 cle of the universe. The number of such messages perceptible is limited 

 only by a most remarkable capacity for differentiating impressions. 

 A thirty-thousandth of a square millimeter of retina is capable of 

 isolating and preserving the identity of a particular sensation, and of 

 appreciating a radically different sensation ten times in each second. 

 This ability of the retina to differentiate impressions is of value 

 only when connected with a correspondingly minute accuracy in the 

 assorting of the rays falling upon it; and this minute accuracy in the 

 assortment of these rays depends on the perfection of the dioptric 

 apparatus of the eye. Its capacity for successive impressions depends 

 on the rapidity of renewal of physical and chemic conditions upon 

 the perfection of its nutrition. To supply and maintain by most 

 delicate adjustments and compensations, these two things, the 

 dioptric assortment of rays and the nutritive conditions of vision, 

 are the essential purposes of the eyeball and its appendages. 



On the one side ophthalmology extends to include the whole science 

 of optics. Optical instruments are but artificial extensions of the 



