SINGLE VISION. 665 



the inner portion of the other eye ; or a of the eye A (fig-. 

 1 88) with a of the eye B. The upper part of one retina is- 

 also identical with the 

 upper part of the other ; 

 and the lower parts of 

 the two eyes are iden- 

 tical with each other. 



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 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 lumi- 

 nous 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 sid& 

 a of the eye B, a single spectrum is produced, and is ap- 

 parent at the extreme right of the field of vision ; if upon 

 the point Z> of one eye, and the point b' of the other, a 

 single spectrum is seen to the extreme left. 



The spheres of the two retina3 may, therefore, be re- 

 garded 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 



