16 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
through the pseudoscope. When an object approaches, the mag-nitude of its picture 
on the retinae increases as in ordinary vision, but the inclination of the optic axes, 
instead of increasing-, becomes less, as I have already explained. Now an enlarge- 
ment of the picture on the retina invariably suggests approach, and a less conver- 
gence of the optic axes indicates that the object is at a greater distance ; and we 
have thus two contradictory suggestions. Hence, if two objects be placed side by 
side at a certain distance before the eyes, and one of them be moved forwards, so as 
to vary its distance from the other, its continually enlarging picture on the retina 
makes it appear to come towards the eyes, as it actually does, while at the same time 
it appears at every step at a greater distance beyond the fixed object ; from one sug- 
gestion the object appears to approach, from the other to have receded. 1 again ob- 
serve that retinal magnitude does not itself suggest distance, but from its changes 
we infer changes of distance, 
1 have hitherto only described the pseudoscope constructed with two reflecting 
prisms. This is the most convenient apparatus for effecting the conversion of 
distance and relief that has occurred to me ; but other means may be employed, 
which I will briefly mention. 
1st. Two plane mirrors are placed together so as to form a very obtuse angle 
towards the eyes of the observer ; immediately before them the object is to be placed 
at such distance that a reflected image shall appear in each mirror. The eyes being 
placed before and a little above the object, must be caused to converge to a point 
between the object and the mirrors ; the right-hand image of the left eye will then 
unite with the left-hand image of the right eye, and the converse relief will be per- 
ceived. The disadvantages of this method are that only particular objects can be 
examined, and it requires a painful adaptation of the eye to distinct vision. 
2ndly. Place between the object and each eye a lens of small focal distance, and 
adjust the distances of the object and the lenses so that distinct inverted images of 
the object shall be seen by each eye ; on directing the eyes to the place of the object 
the two images will unite, and the converse .relief be perceived. As the rays of light 
proceeding from the images have a greater divergence than those which would pro- 
ceed from the point to which the optic axes are directed, long-sighted persons will 
see the binocular image more distinctly by wearing a pair of short-sighted spectacles. 
In this experiment the field of view is very small on account of the distance at which 
it is necessary to place the lenses from the eyes ; but I have been enabled in this 
manner to see beautifully the converse relief of a small ivory bust and of other small 
objects, which hovvever should be inverted in order to see them direct. 
3rdly. The inverted images of the lenses, instead of being received immediately by 
the eyes as just described, may be thrown on a plate of ground glass as in the case 
of the ordinary camera-obscura, and may be then caused to unite by the means 
employed in any form of the refracting stereoscope. 
