MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 385 



line inclined some degrees from the perpendicular (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 dif- 

 ferent eye, will coincide, and the resultant perspective 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 direction of 

 the left eye in which it is situated, with the plane of visual direction of the right eye, 

 which contains the strong vertical line. 



This experiment affords another proof that there is no necessary physiological con- 

 nection between the corresponding points of the two retinse, — a doctrine which has 

 been maintained by so many authors. 



§ 13. Binocular Vision of Images of different Magnitudes. 



We will now inquire what effect results from presenting similar images, differing 

 only in magnitude, to analogous parts of the two retinse. For this purpose two 

 squares or circles, differing 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, notwithstanding this difference, they coalesce and occasion a single 

 resultant perception. The limit of the difference of size within which the single ap- 

 pearance subsists may be ascertained by employing two images of equal magnitude, 

 and causing one of them to recede from the eye while the other remains at a con- 

 stant distance ; this is effected merely by pulling out the sliding board C (fig. 8.) 

 w^hile the other C remains fixed, the screw having previously been removed. 



Though the single appearance of two images of different size is by this experiment 

 demonstrated, the observer is unable to perceive what difference exists between the 

 apparent magnitude of the binocular image and that of the two monocular images; 

 to determine this point the stereoscope must be dispensed with, and the experiment so 

 arranged that all three shall be simultaneously seen; which may be done in the foU 

 lowing manner : — The two drawings being placed side by side on a plane before the 

 eyes, the optic axes must be made to converge to a nearer point as at fig. 4., or to a 

 more distant one as at fig. 3., until the three images are seen at the same time, the 

 binocular image in the middle, and the monocular images at each side. It will thus 

 be seen that the binocular image is apparently intermediate in size between the two 

 monocular ones. 



If the pictures be too unequal in magnitude, the binocular coincidence does not 



MDCCCXXXVIII. 3 D 



