DIOPTRICS OF THE EYE. 



311 



distance coincides with the retina, since the refracting surfaces 

 in the normal resting eye are so formed that parallel rays (rays 

 from distant objects) are brought to a focus on the retina. To 

 show the formation of the image of an external object on the retina 

 it suffices, therefore, to use a construction such as is represented in 

 Fig. 128. Secondary axes are drawn from the limiting points of the 

 object A and B through the nodal point. Where these axes 

 cut the retina the retinal image of the object will be formed. That 

 is, all the rays of light proceeding from A that penetrate the eye will 

 be focused at a, and all proceeding from B at b. The image on 

 the retina will therefore be inverted and will be smaller than the 

 object. The angle formed at the nodal point by the lines A-n and 

 B-n is known as the visual angle; it varies inversely with the dis- 

 tance of the object from the eye. 



The Inversion of the Image on the Retina. Although the 

 images of external objects on the retina are inverted, we see them 

 erect. This fact is easily understood when we remember that our 

 actual visual sensations take place in the brain and that the pro- 

 jection of these sensations to the exterior is a secondary act that has 

 been learned from experience. Experience has taught us to project 

 the visual sensation arising from the stimulation of any given point 

 on the retina to that part 

 of the external world 

 from which the stimulus 

 arises that is, to the 

 luminous point giving 

 origin to the light rays. 

 According to the physi- 

 cal principles described 

 above, the image of such 

 a point must be formed 

 on the retina where the 

 secondary axis from that 



point through the nodal point touches the retina. In projecting this 

 retinal stimulus outward to its source, therefore, we have learned 

 to project it back, as it were, along the line of its secondary axis. 

 In Fig. 128 the retinal stimulus at a is projected outward along 

 the line a-n-A, and to such a distance as, from other sources, we 

 estimate the object A to be. This law of projection is fixed by 

 experience, but it implies, as will be noted, that we are conscious 

 of the differences in sensation aroused by stimulation of different 

 parts of the retina. Considering the retina as a sensory surface, 

 like the skin, for instance, each point, speaking in general terms, 

 may be assumed to be connected with a definite portion of the 

 cortex, and the sensation aroused by the stimulation of these dif- 

 ferent points must differ to some extent in consciousness, each has 



Fig. 128. Diagram to illustrate the construc- 

 tion necessary to determine the location and size of 

 the retinal image. 



