Prof. Wheatstone on the Physiology of Vision. 521 



the most distant, when the eyes are directed horizontally towards 

 it, the rule will appear exactly in the converse position. If the 

 rule be now removed lower down in the same vertical plane, its 

 inclination remaining unchanged, so that to look upon it the 

 plane of the optic axes must be inclined 45°, it will appear un- 

 altered in position, because its two pictures are parallel on the 

 retina?, and the optic axes would require the same convergence 

 to make the upper and lower ends coalesce. The rule being 

 removed still lower down, instead of its position being apparently 

 reversed, it will appear to have a greater inclination on the same 

 side than the object itself has. In the first case the more distant 

 end is actually furthest from the eyes ; in the second, the near 

 and remote ends are equally distant ; and in the third the nearest 

 end is most distant. 



Attention to what I have just stated will explain many ano- 

 malous circumstances which occur when the eyes are differently 

 directed towards the same object. It may also be necessary to 

 remark, that the conversion of distance takes place only within 

 those limits in which the optic axes sensibly converge, or the 

 pictures projected on tbe retina? are sensibly dissimilar. Beyond 

 this range there is no mutual transposition of the apparent 

 distances of objects with the pseudoscope ; a distant view there- 

 fore appears unchanged. 



Some very paradoxical results are obtained when objects in 

 motion are viewed through the pseudoscope. When an object 

 approaches, the magnitude of its picture on the retina? increases 

 as in ordinary vision ; but the inclination of the optic axes, 

 instead of increasing, becomes less, as I have already explained. 

 Now an enlargement of the picture on the retina invariably sug- 

 gests approach, and a less convergence 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 eveiy step at a greater distance beyond the 

 fixed object ; from one suggestion the object appears to approach, 

 from the other to have receded. I again observe that retinal 

 magnitude does not itself suggest distance, but from its changes 

 we infer changes of distance. 



I have hitherto only described the pseudoscope constructed 

 with two reflecting prisms. This is the most convenient appa- 

 ratus for effecting the conversion of distance and relief that has 

 occurred to me ; but other means may be employed, which I 

 will briefly mention. 



