THE SENSE OF VISION. 



751 



points of the object through the above-mentioned nodal point and to prolong 

 them till they strike the retina. It is evident that the size of the retinal image 

 will be as much smaller than that of the object as the distance of the nodal 

 point from the retina is smaller than its distance from the object. 



According to the figures above given, the nodal point is about 7.2 milli- 

 meters behind the anterior surface of the cornea and about 15.0 millimeters in 

 front of the retina. Hence the size of the retinal image of an object of known 

 size and distance can be readily calculated a problem which has frequently to be 

 solved in the study of physiological optics. The construction given in Figure 

 216 shows that from all external objects inverted images are projected upon the 

 retina, and such inverted images can actually be seen under favorable condi- 

 tions. If, for instance, the eye of a white rabbit, which contains no choroidal 

 pigment, be excised and held with the cornea directed toward a window or 

 other source of light, an inverted image of the luminous object will be seen 

 through the transparent sclerotic in the same way that one sees an inverted 

 image of a landscape on the ground-glass plate of a photographic camera. 

 The question is often asked, " Why, if the images are inverted in the retina, 

 do we not see objects upside down ?" The only answer to such a question is 

 that it is precisely because images are inverted on the retina that we do not see 

 objects upside down, for the eye has learned through lifelong practice to asso- 

 ciate an impression made upon any portion of the retina with light coming 

 from the opposite portion of the field of vision. Thus if an image falls upon 

 the lower portion of the retina, our experience, gained chiefly through mus- 

 cular movements and tactile sensations, has taught us that this image must cor- 

 respond to an object in the upper portion of our field of vision. In whatever 

 way the retina is stimulated the same effect is produced. If, for instance, 

 gentle pressure is made with the finger on the lateral portion of the eyeball 

 through the closed lids a circle of light known as a phosphene immediately 

 appears on the opposite side of the eye. Another good illustration of the 

 same general rule is found in the effect of throwing a shadow upon the retina 

 from an object as close as possible to the eye. For this purpose place a card 



B p 



FIG. 217. Diagram illustrating the projection of a shadow on the retina. 



with a small pin-hole in it in front of a source of light, and three or four 

 centimeters distant from the eye. Then hold some object smaller than the 

 pupil e. g. the head of a pin as close as possible to the cornea. Under these 

 conditions neither the pin-hole nor the pin-head can be really seen i. e. they 



