MR. WHEATSTONE ON THE PHYSIOLOGY OP VISION. 
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sents all the possible changes of the two binocular pictures; those marked N show 
the normal relief, and those marked C the converse relief. 
But it may be asked why, if the reflection or inversion of the binocular pictures of 
an object gives rise to the mental idea of the converse relief, the same converse relief 
is not observed when the object itself is reflected in a mirror, or inverted. The 
reason is this ; that in the former cases the projections to each eye are separately 
reflected or inverted, still remaining presented to the same eye, whereas, by the re- 
flection or inversion of the object itself, not only are the projections reflected or 
inverted, but they are also transposed from one eye to the other ; and these circum- 
stances occurring simultaneously reproduce the normal relief. 
Fig. 6 will render this evident in the case of reflexion : A is the object, B its re- 
flexion in the mirror CD ; RB and LB are the directions in which the right and left 
eyes view the reflected image respectively, and /A and rA the directions in which the 
eyes would view the corresponding face of the object directly. 
In the case of an inverted object, it is obvious that that projection which was before 
seen by the right eye must be seen by the left eye, and the contrary. 
It is possible to make this normal or converse relief appear while one of the pic- 
tures remains constantly presented to the same eye. This result may be thus obtained. 
Having taken a photograph of the object, which should be one the converse of 
which has a meaning, take two others at the same angular distance (say 18°), one on 
the right side, the other on the left side of the original. Of the three pictures thus 
taken, if the middle one be presented to the right eye, and the left picture to the left 
eye, a normal relief will be seen ; but if the right picture be presented to the left eye, 
the other remaining unchanged, a converse relief will be seen. In like manner, if 
the middle picture be presented to the left eye, and the right picture to the right eye, 
a normal relief will appear ; but if the left picture be presented to the right eye, the 
converse relief will present itself. It must be observed, that the normal and converse 
reliefs, when the same picture remains presented to the same eye, belong to two dif- 
ferent positions of the object. 
§23. 
Hitherto I have taken into consideration only those cases of the conversion of 
relief which are exhibited by binocular pictures in the stereoscope, when they are 
transposed, reflected or inverted ; I shall now proceed to show how phenomena of 
the same kind may be elicited by regarding objects themselves, by means of an in- 
strument adapted for the purpose. As this instrument conveys to the mind false 
perceptions of all external objects, I have called it the Pseudoscope. It is represented 
by fig. 7j and is thus constructed : two rectangular prisms of flint glass, the faces 
of which are 1*2 inch square, are placed in a frame with their hypothenuses parallel, 
and 2T inches from each other; each prism has a motion on an axis corresponding 
with the angle nearest the eyes, so that they may be adjusted that their bases may 
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