12 FORMATION OF RETINAL IMAGES. [BOOK in. 



of the arrow. In a similar way a straight line drawn through the 

 nodal point from the extreme point of the other end of the arrow, 

 and continued until it meets the retina at y> will give us the 

 position of the image of the other end of the arrow ; and in like 

 manner lines drawn from other points of the arrow through the 

 same nodal point will give us the position on the retina of the 

 images of those other points. In this way the construction of the 

 reduced eye enables us to ascertain the position, magnitude and 

 features of the retinal image of an object. 



707. A ray of light, that is to say a series of waves of 

 ether, falling upon a point of the retina stimulates certain 

 structures in the retina and gives rise, as we have said, to visual 

 impulses and so to a sensation of light ; this we may consider as a 

 visual sensation in its simplest form. When a number of different 

 points of the retina are thus stimulated at the same time, as when 

 an image of an external object falls in proper focus on the retina, 

 the total result is a complex group of visual impulses and thus a 

 complex sensation, by which we perceive, as we say, the object ; 

 and we frequently speak of this complex sensation as a visual 

 image corresponding to the retinal image. The term is perhaps 

 not a very desirable one, since it seems to imply an identity 

 between the former which is a psychical matter, and the latter 

 which is a physical matter ; whereas, the one thing we may be 

 sure about is that the psychical thing, though it is a sign and 

 token of, is wholly different from the physical thing. 



It will be as well perhaps thus early to call attention to the 

 fact that, as indeed is shewn in Fig. 137, the image on the retina 

 is an inverted one. What is the upper part of the object in the 

 external world is represented in the lower part of the retinal 

 image, what is on the right-hand side of the object is represented 

 on the left-hand side of the image. In the visual judgment which 

 is based upon the visual sensation, the retinal image is, as it were, 

 reinverted ; we take the left-hand side, or the bottom of the 

 retinal image, as a token or sign of the right-hand side or the top 

 of the object seen. We shall return to this matter later on ; but 

 in studying the dioptrics of the eye this inversion of the retinal 

 image must always be borne in mind. 



