THE EYE AS AN OPTICAL INSTRUMENT. 199 



But, after all it remains a wonderful parodox, that we are 

 so slow to observe these and other peculiarities of vision (such 

 as the after-images of bright objects), so long as they are not 

 strong enough to prevent our seeing external objects. It is a 

 fact which we constantly meet, not only in optics, but in study- 

 ing the perceptions produced by other senses on the conscious- 

 ness. The difficulty with which we perceive the defect of the 

 blind spot is well shown by the history of its discovery. Its 

 existence was first demonstrated by theoretical arguments. 

 While the long controversy whether the perception of light re- 

 sided in the retina or the choroid was still undecided, Mariotte 

 asked himself what perception there was where the choroid is 

 deficient. He made experiments to ascertain this point, and in 

 the course of them discovered the blind spot. Millions of men 

 had used their eyes for ages, thousands had thoiight over the 

 nature and cause of their functions, and, after all, it was only 

 by a remarkable combination of circumstances that a simple 

 phenomenon was noticed which would apparently have revealed 

 itself to the slightest observation. Even now, anyone who tries 

 for the first time to repeat the experiment which demonstrates 

 the existence of the blind spot, finds it difficult to divert his 

 attention from the fixed point of clear vision, without losing 

 sight of it in .the attempt. Indeed, it is only by long practice in 

 optical experiments that even an experienced observer is able, as 

 soon as he shuts one eye, to recognise the blank space in the field 

 of vision which corresponds to the blind spot. 



Other phenomena of this kind have only been discovered by 

 accident, and usually by persons whose senses were peculiarly 

 acute, and whose power of observation was unusually stimu- 

 lated. Among these may be mentioned Goethe, Purkinje, 1 and 

 Johannes Miiller. 2 When a subsequent observer tries to repeat 

 on his own eyes these experiments as he finds them described, 

 it is of course easier for him than for the discoverer ; but even 



1 A distinguished embryologist, for many years professor at Breslau : he died 

 at Prague, 1869, set. 82. 



2 A great biologist, in the full sense of the term. He was professor of 

 physiology at Berlin, and died 18J8, net. 57. His Manual of Physiology was 

 translated into English by the late Dr. Baly. TR. 



