320 Mr. W. Le Conte Stevens on 



alone. They indicate a closer relation between the nerves of 

 the ciliary muscle and those of the retina than has been com- 

 monly supposed. Through the sympathetic system of nerves 

 the mere action of light on the retina excites reflex muscular 

 contraction of the iris. Without the agency of light, this 

 may be effected by ciliary contraction alone. It is by no 

 means impossible that through this same sympathetic system 

 the impression conveyed to the brain may be modified by such 

 muscular contraction, while the retinal area impressed remains 

 unchanged. The interpretation is unconscious and indepen- 

 dent of the will ; it is dependent upon the excitement of 

 several different nerves. The thing interpreted is the product 

 of forces from without that operate in accordance with well- 

 known mathematical laws. The velocity of propagation of a 

 nerve-impression has been approximately determined byHelm- 

 holtz; but beyond this there has been little success in bring- 

 ing the physiology of sensation within the domain of mathe- 

 matics. 



That the interpretation of retinal impressions should be more 

 readily modified by association with muscular action when 

 they are produced upon central rather than marginal portions 

 of the retina, is entirely in accordance with the empiristic 

 theory, which Helmholtz, Briicke, and other German physio- 

 logists have upheld. In all ordinary vision the attention is 

 habitually given almost exclusively to objects corresponding 

 to retinal images on or near the yellow spot. If an}- condi- 

 tions are imposed that tend to induce misinterpretation of our 

 sensations, it might be expected that the most sensitive parts 

 of the retina would be most affected. It is impossible to 

 prove that there is any peculiar necessary relation between 

 the nature of a retinal sensation and the distance of the object 

 pictured, or any innate power to recognize distance. Visual 

 experience begins in infancy, long before the power to ana- 

 lyze sensations ; and, in our interpretation of these, associa- 

 tion is the chief determinant. Strong ciliary contraction is 

 habitually, though unconsciously, associated with nearness of 

 the object viewed; and this is pictured upon or very near the 

 yellow spot of the retina. No one whose eyes are perfectly 

 healthy has any consciousness of possessing a retina except in 

 relation to external objects of sight, or a ciliary muscle except 

 in relation to variation in distance of the point upon which 

 attention is fixed through the medium of a simultaneous 

 retinal sensation. We might expect, therefore, that unusual 

 muscular conditions would cause misinterpretation regarding 

 the whole field of view, but especially those parts which cor- 

 respond to the central portion of the retina. This reasoning 



