Prof. Wheatstone on the Physiology of Vision. 257 



tention ; the series of successive projections cannot then be re- 

 ferred to any figure to which they are all common, and the 

 skeleton figure will appear to be continually undergoing a change 

 of shape. 



§12- 



I have given ample proof that objects whose pictures do not 

 fall on corresponding points of the two retinae may still appear 

 single. I will now adduce an experiment which proves that 

 similar pictures falling on corresponding points of the two re- 

 tinae may appear double and in different places. 



Present, in the stereoscope, to the right eye a vertical line, 

 and to the left eye a line inclined some degrees from the per- 

 pendicular (fig. 23) ; the observer will then perceive, as formerly 

 explained, a line, the extremities of which appear at different 

 distances before the eyes. Draw on the left-hand figure a faint 

 vertical line exactly corresponding in position and length to that 

 presented to the right eye, and let the two lines of this left-hand 

 figure intersect each other at their centres. Looking now at 

 these two drawings in the stereoscope, the two strong lines, each 

 seen by a different eye, will coincide, and the resultant perspec- 

 tive line will appear to occupy the same place as before ; but 

 the faint line which now falls on a line of the left retina, which 

 corresponds with the line of the right retina on which one of 

 the coinciding strong lines, viz. the vertical one, falls, appears in 

 a different place. The place this faint line apparently occupies 

 is the intersection of that plane of visual directions of the left 

 eye in which it is situated, with the plane of visual directions of 

 the right eye, which contains the strong vertical line. 



This experiment affords another proof that there is no neces- 

 sary physiological connection between the corresponding points 

 of the two retinae, — a doctrine which has been maintained by 

 so many authors. 



§ T3. Binocular Vision of Images of different Magnitudes. 



We will now inquire what effect results from presenting simi- 

 lar images, differing only in magnitude, to analogous parts of 

 the two retinae. For this purpose two squares or circles, differ- 

 ing obviously but not extravagantly in size, may be drawn on 

 two separate pieces of paper, and placed in the stereoscope so 

 that the reflected image of each shall be equally distant from the 

 eye by which it is regarded. It will then be seen that, not- 

 withstanding this difference, they coalesce and occasion a single 

 resultant perception. The limit of the difference of size within 

 which the single appearance subsists may be ascertained by em- 

 ploying two images of equal magnitude, and causing one of them 



Pfdl Mag. S. 4. Vol. 3. No. 18. April 1852. S 



