522 THE SENSE OF SIGHT. 



This is proved by a single experiment. Pressure upon any 

 part of the ball of the eye, so as to affect the retina, produces 



a luminous circle, seen at the 

 FIG. IBS. opposite side of the field of 



vision to that on which the 

 pressure is made. If, now, in 

 a dark room, we press with 

 the finger at the upper part 

 of one eye, and at the lower 

 part of the other, two lumin- 

 ous circles are seen, one above 

 the other; so, also, two figures 

 are seen when pressure is made 

 simultaneously on the two outer or the two inner sides of both 

 eyes. It is certain, therefore, that neither the upper part of 

 one retina and the lower part of the other are identical, nor 

 the outer lateral parts of the two retinae, nor their inner lateral 

 portions. But if pressure be made with the fingers upon both 

 eyes simultaneously at their lower part, one luminous ring is 

 seen at the middle of the upper part of the field of vision ; if 

 the pressure be applied to the upper part of both eyes, a single 

 luminous circle is seen in the middle of the field of vision 

 below. So, also, if we press upon the outer side a of the eye 

 A, and upon the inner side a' of the eye B, a single spectrum 

 is produced, and is apparent at the extreme right of the field 

 of vision ; if upon the point b of one eye, and the point b' of 

 the other, a single spectrum is seen to the extreme left. 



The spheres of the two retinae may, therefore, be regarded 

 as lying one over the other, as in c, Fig. 188 ; so that the left 

 portion of one eye lies over the identical left portion of the 

 other eye, the right portion of one eye over the identical right 

 portion of the other eye ; and with the upper and lower por- 

 tions of the two eyes, a lies over a', b over b', and c over c'. 

 The points of the one retina intermediate between a and c, are 

 again identical w r ith the corresponding points of the other 

 retina between a' and c' ; those between b and c of the one 

 retina, with those between b' and c' of the other. In short, all 

 other parts are non-identical : and, when they are excited to 

 action, the effect is the same as if the impressions were made 

 on different parts of the same retina : and the double images 

 belonging to the eyes A and B, are seen at exactly the same 

 distance from each other as exists between the image of the 

 eye A and the part of the retina of the eye A which corre- 

 sponds to, or is identical with, the seat of the second image in 

 the eye B ; or, to return to the figure already used in illustra- 

 tion (Fig. 188), if a of one eye be affected, and b' of the other, 



