Chap, hi.] SIGHT. 845 



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. 



§ 529. 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. 142, 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. 



