PHYSIOLOGICAL OPTICS. 147 



and the radius of curvature of each part is known and the surfaces, although 

 not accurately centred, are sufficiently nearly so to make it possible to find 

 mathematically a single surface which represents the whole. This is used in the 

 construction of the SCHEMATIC EYE (Fig. 41). The single refracting surface is made 

 to lie a few millimetres behind the real cornea. In the unaccommodated eye 

 distant objects are clearly seen, which must mean that the rays from them, which 

 are parallel or nearly so, form sharp images on the light-sensitive surface. The 

 refracting surface of the schematic eye, to represent this, has its principal focal 

 plane on the retina. From a distant point object situated in any part of the 

 visual field, we know the course of one ray, that which is directed straight for the 

 nodal point of the simplified eye and which is not changed in its direction before 

 reaching the retina. By the previous argument we have shown that all rays from 

 such a point meet at the retina. Therefore to find the image on the retina of any 

 object, all we need to do is to draw straight lines from its limiting points through 

 the nodal point of the schematic eye. 



FIG. 41. The formation of an image by the eye as represented by the schematic eye. 



It will be seen that according to the foregoing construction the 

 retinal image is an inverted one. That we interpret this to our- 

 selves as an erect picture of the object may at first sight appear con- 

 fusing. It should be remembered, however, that we are not born 

 with the faculty of associating stimulation of any particular part 

 of the retina with the presence of some object in a particular part 

 of the external world. We learn this in infancy through our other 

 senses, mainly the sense of touch. Because a point on one side of 

 the retina is as a matter of fact always affected by light from some 

 object which we know by our other senses to be on the opposite 

 side of the field, we form the habit of interpreting the stimulation 

 in this way. We associate stimulation of the right side of the retina 

 with the presence of something in the left side of the visual field 

 and so on. To put the matter more generally, we refer stimulation 

 of any part of the retina to the presence of some object in the out- 



