14 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
form or its converse is perceived. This mental attention is involuntary ; no imme- 
diate effort of the will can call up one idea while the other continues to present itself, 
though the transition may be facilitated by intentionally removing some of the signs 
which suggest the preponderating idea ; thus the converse form being perceived, 
closing either eye will most frequently cause an instant reversion to the normal form ; 
and always, if the monocular signs of relief are sufficiently suggestive. 
I know of nothing more wonderful, among the phenomena of perception, than the 
spontaneous successive occurrence of these two very different ideas in the mind, 
while all external circumstances remain precisely the same. Thus a small statuary 
group, an elegant and beautiful object, without any apparent cause becomes con- 
verted into another totally dissimilar object uncouth in appearance, and which gives 
rise to no agreeable emotions in the mind ; yet in both cases all the sensations that 
intervene between objective reality and ideal conception continue unchanged. 
The effects of the pseudoscope I have already mentioned, may be strictly called 
conversions of relief, because the illusive appearance is in each case the converse 
impression of the relief of the real object. If, however, the object consists of parts 
detached from and behind each other, the preceding term is inappropriate to denote 
the effects which result, but the more general expression conversion or inversion of 
distance may be employed to designate them. I proceed to call attention to a few 
such effects. 
Skeleton figures of geometrical solids, as cubes, pyramids, &c., readily show their 
converse. 
Two objects at different distances, being simultaneously regarded, the most remote 
will appear the nearest and the nearest the most remote. 
An ivory foot rule, held immediately before the eyes a little inclined to the horizon 
with its remote end elevated, appears inclined in the opposite way, its nearer end ele- 
vated, and as if the observer were looking at its lower surface. Its form also under- 
goes a change. Since the nearest end, the retinal magnitude of which is the largest, 
appears farthest from the eyes, and the nearest end, the retinal magnitude of which 
is greatest, appears near the eyes, the rule will no longer be perceived to be rectan- 
gular, but trapezoidal. If the rule be placed horizontally, and it be regarded with 
the pseudoscope at an angle of 45°, it will appear with the form just described stand- 
ing vertically. 
Any object placed before the wall of a room will appear behind the wall, and as if 
an aperture of the proper dimensions had been made in the wall to allow it to be 
seen; if the object be illuminated by a candle, its shadow will appear as far before 
the object as in reality it is behind. 
The appearance of a plant is very remarkable ; as the branches which are farthest 
from the eye are perceived to be the nearest, those parts which are actually obscured 
by the branches before them, appear broken away and allow the parts apparently be- 
