THE EYE AS AN OPTICAL INSTRUMENT. 225 



external objects. It is a fact which we constantly meet, 

 not only in optics, but in studying the perceptions pro- 

 duced by other senses on the consciousness. The diffi- 

 culty 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 resided in the retina or the choroid was still unde- 

 cided, 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 thought over the nature and cause 

 of their functions, and, after all, it was only by a remark- 

 able combination of circumstances that a simple pheno- 

 menon 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 diffi- 

 cult 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 stimulated. Among these may be mentioned 

 Goethe, Purkinje,' and Johannes Miiller.^ When a sub- 



' 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 1858, set. 57. His Manual of Physiology 

 was translated into English by the late Dr. Baly. — Tk. 



