308 THE SPECIAL SENSES. 



situated on this line. This law of projection is so firmly fixed that 

 if a given point in the retina is stimulated in some unusual way 

 we still project the resulting sensation outward according to the 

 law, and thus make a false projection and interpretation. For 

 instance, if the little finger is inserted into the inner and lower angle 

 of the eye and is pressed upon the eyeball the edge of the retina is 

 stimulated mechanically. One experiences, in consequence, a 

 visual sensation, known as a phosphene, consisting of a dark-blue 

 spot surrounded by a light halo. This sensation, however, is 

 projected out toward the upper and outer angle of the eye, accord- 

 ing to the law of projection, since normally this part of the retina 

 is only stimulated by light coming from such a direction. A similar 

 error in projection is obtained by holding objects so close to the eye 

 that a physical inverted image cannot be formed, but only an erect 

 shadow image. This experiment may be performed as follows: 

 Hold the head of a pin close to the eye, and, in order that a sharp 

 shadow may be thrown, allow the light to fall on this pin through 

 a pinhole in a card held somewhat farther from the eye. By this 

 means an erect shadow of the pin, lying in the circle of light from 

 the hole, will be thrown on the eye. This shadow image will be 

 projected outward according to the usual law, and consequently 

 wiD appear inverted. 



The Size of the Retinal Image. — ^The size of the image of an 

 object on the retina may be reckoned easily, provided the size of the 

 object and its distance from the eye is known. As will be seen from 

 the construction given in Fig. 128, the triangles A-n-B and a-n-b are 

 symmetrical ; consequently we have the ratio : 



A-B : a-b : : A-n : a-n or 



A-B A-n ,. ^ . 



— p- = : that IS 



a-b a-n 



Size of object Distance of object from nodal point. 



Size of image "" Distance of image from nodal point. 



As was stated above, the distance of the im.age from the nodal 

 point — that is, the distance of the retina from the nodal point — 

 may be placed at 15.5 or 15 mms. Consequently, three of the factors 

 in the above equation being known, it is easily solved for the un- 

 known factor — namely, the size of the image on the retina. To 

 take a concrete example; suppose it is desired to know the size on 

 the retina of the image made by an object 120 feet high at a distance 

 of one mile (5280 feet). If we designate the size of the image as x 

 and substitute the known values for the other terms of the equation, 

 tve have — = ^S^, or a; = 0.341 mm., which is about the diam- 



x 15 ' 



eter of the fovea centralis. The retinal image of the object in 

 this case would be, in round numbers, about yr-^Vir-ir of the actual 

 height of the object. 



