404 ELEMENTARY PHYSIOLOGY 



2. The Eye as a Water-Camera.— The impact of the 

 ethereal vibrations upon the sensory expansion, or essential 

 part of the visual apparatus alone, is sufficient to give rise 

 to all those feelings, which we term sensations of light and 

 of colour, and further to that feeling of outness which 

 accompanies all visual sensation. But, if the retina had 

 a simple transparent covering, the vibrations radiating 

 from any number of distinct luminous points in the 

 external woi-ld would affect all parts of it ecjually, and 

 therefore the feeling aroused would be that of a generally 

 diffused luminosity. There would be no separate feeling 

 of light for each separate radiating point, and hence no 

 correspondence between the vi.sual sensations and the 

 radiating points which aroused them. 



It is obvious that, in order to produce this correspond- 

 ence, or, in other words, to have distinct vision, tlie 

 essential condition is, that distinct luminous points in the 

 external Avorld shall be represented by distinct feelings of 

 light. And since, in order to jjroduce tliese distinct 

 feelings, vibrations must fall on separate parts of the 

 retina, it follows that, for the production of distinct 

 vision, some apparatus must be interposed between the 

 retina and the external world, by the action of which 

 distinct luminous points in the latter shall be represented 

 by corresponding points of light on the retina. 



In the eye of man and of the higher animals, this acces- 

 sory appAratus of vision is represented by structures which, 

 taken together, act as a biconvex lens, composed of sub- 

 stances which have a much greater refractive power than 

 the air by which the eye is sun-ounded ; and which throw 

 upon the retina luminous points, which correspond in 

 number, and in position relatively to one another, with 

 those luminous points in tlie external world from which 

 ethereal vibrations proceed towards the eye. The lumin- 

 ous points thus thrown upon the retina form a picture 

 of the external world — a picture being nothing but lights 

 and shadows, or colours, arranged in such a way as to 



