516 Prof. Wheatstone on the Physiology of Vision. 



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 reflexion 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 cir- 

 cumstances occurring simultaneously reproduce the normal relief. 



Fig. 6 will render this evident in the case of reflexion : A is 

 the object, B its reflexion 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 pro- 

 jection 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 pictures 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 di- 

 stance (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 con- 

 verse 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 pre- 

 sented 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 different 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 in- 

 verted ; I shall now proceed to show how phenomena of the 

 same kind may be elicited by regarding objects themselves, by 

 means of an instrument adapted for the purpose. As this in- 

 strument conveys to the mind false perceptions of all external 

 objects, I have called it the Pseudoscope. It is represented 

 by fig. 7, 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 2"1 inches from 

 each other ; each prism has a motion on an axis corresponding 



