THE STEREOSCOPE. 



render us unconscious of them, and we imagine that shape, dis- 

 tance, and position are the immediate subjects of visual percep- 

 tion, instead of being consequences deduced from a set of percep- 

 tions of a wholly different kind. 



2. In drawing and painting, the effects of perspective and relief 

 are therefore reproduced* by transferring to the canvas the same 

 outlines and the same varieties of light and shade, which the 

 objects delineated really present to the eye, and when this has 

 been accomplished with the necessary degree of fidelity and pre- 

 cision, the same impression of distance, perspective, and relief is 

 produced, as that which would be received from the immediate 

 view of the objects themselves which are delineated. 



3. In certain exceptional cases, however, a class of visual phe- 

 nomena is manifested which are quite independent of mere 

 outline and varieties of light and shadow, and which no effort of 

 art can transfer to canvas. Inasmuch, also, as these phenomena, 

 like those already mentioned, are optical effects of distance, form, 

 and position, they become, like the others, indications by which 

 the mind judges of the relative forms and positions of the objects 

 which produce them. Phenomena of this class are manifested, 

 when the objects viewed are placed so near the observer, as to 

 have sensible binocular parallax. The aspects under which they 

 are seen in this case by the two eyes, right and left, are different. 

 Certain parts are visible to each eye which are invisible to the 

 other, and the relative positions in which some parts are seen by 

 one eye, differ from those in which the same parts are seen by the 

 other eye. This difference of aspect and apparent position, arises 

 altogether from the different position of the two eyes in relation 

 to the objects. It is a phenomenon, therefore, which can never 

 be developed, in the case of objects whose distance bears a large 

 proportion to the distance between the eyes, because there is no 

 sensible difference between the aspects under which such objects 

 are viewed by the one eye and the other. The phenomenon, 

 therefore, can only be manifested in relation to objects, whose 

 distance from the observer is a small multiple of the distance 

 between the eyes. 



4. To render this more clear, let us imagine a bust presented to 

 an observer at a distance of a few feet, the face being turned 

 obliquely so that one side is presented more to view than the 

 other. Supposing the side which is turned towards the observer 

 to be on his right, it is evident that the nose will intercept, more 

 or less, the view of the side of the face which is on his left, but 

 the part which it thus intercepts will not be the same for both 

 eyes. It will evidently intercept more from the right than the 

 left eye. On the other hand, the right eye will see a part of the 



