THE PERCEPTION OF SIGHT. 241 



physiological problems has only become possible in very modern 

 times, particularly since we have gained more accurate know- 

 ledge of the movements of the eye. The invention of the stereo- 

 scope by Wheatstone (p. 249) made the difficulties and imper- 

 fections of the Innate Theory of sight much more obvious than 

 before, and led to another solution which approached much 

 nearer to the older view, and which we will call the Empirical 

 Theory of Vision. This assumes that none of our sensations 

 give us anything more than ' signs ' for external objects and 

 movements, and that we can only learn how to interpret these 

 signs by means of experience and practice. For example, the con- 

 ception of differences in locality can only be attained by means 

 of movement, and, in the field of vision, depends upon our expe- 

 rience of the movements of the eye. Of course this Empirical 

 Theory must assume a difference between the sensations of 

 various parts of the retina, depending upon their local diffe- 

 rence. If it were not so, it would be impossible to distinguish 

 any local difference in the field of vision. The sensation of red, 

 when it falls upon the right side of the retina, must in some 

 way be different from the sensation of the same red when it 

 affects the left side ; and, moreover, this difference between the 

 two sensations must be of another kind from that which we 

 recognise when the same spot in the retina is successively affected 

 by two different shades of red. Lotze 1 has named this diffe- 

 rence between the sensations which the same colour excites when 

 it affects different parts of the retina, the local sign of the sen- 

 sation. We are for the present ignorant of the nature of this 

 difference, but I adopt the name given by Lotze as a convenient 

 expression. While it would be premature to form any 

 further hypothesis as to the nature of these ' local signs,' there 

 can be no doubt of their existence, for it follows from the fact 



of the Royal College of Physicians, was born in 1384, and died in 1760. Besides 

 works on the Contraction of the Heart, on Vis Viva, &c., h*e published, in 1738, 

 a treatise on Distinct and Indistinct Vision. TR. 



1 Rudolf Hermann Lotze, Professor in the University of Gottingen, origin- 

 ally a disciple of Herbart (v. supra), author of Allgemeine Physiologic des 

 menschlichen Korpers, 1851. TR. 



I. B 



