766 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



Fig. 273, the upper part of figures A, B, when combined in the Stereoscope, 

 excite the idea of a projecting truncated pyramid, with the small square in 

 the centre, and the four sides sloping equally away from it ; whilst the lower 

 pair of figures, c, D, which are the same as the upper, but transferred to the 

 opposite sides, no less vividly bring before the mind the visual conception 

 of a receding pyramid, still with the small square in the centre, and the four 

 sides sloping equally towards it. Professor Wheatstone has further shown, 

 by means of the Stereoscope, that similar images, differing to a certain ex- 

 tend in magnitude, when presented to the corresponding parts of the two 

 retiuse, give rise to the perception of a single object, intermediate in size 

 between the two monocular pictures. Were it not for this, objects would 

 appear single only when at an equal distance from both eyes, so that their 

 pictures upon the retiuse are of the same size ; which will not happen unless 

 they are directly in front of the median line of the face. Again, if pictures 



of dissimilar objects be simulta- 

 neously presented to the two eyes, 

 the consequence will be similar 

 to that which is experienced 

 when the rays come to the eye 

 through two differently colored 

 media ; the two images do not 

 coalesce, nor do they appear per- 

 manently superposed one upon 

 the other ; but at one time one 

 image predominates to the exclu- 

 sion of the other, and then the 

 other is seen alone ; and it is 

 only at the moment of change 

 that the two seem to be inter- 

 mingled. It does not appear to 

 be in the power of the Will, 

 Professor Wheatstoue remarks, to determine the appearance of either ; but 

 if one picture be more illuminated than the other, it will be seen during a 

 larger portion of the time. If, however, the differences in the two pictures 

 be such that the Mind can reconcile them, an intermediate conception is 

 formed; thus if two photographic portraits be taken at the proper angle for 

 the Stereoscope, not simultaneously but consecutively, and the "sitter" alter 

 his expression in the interval, so that one of the portraits represents him 

 with a smile, and the other with a frown, the Stereoscopic image will present 

 an intermediate expression of placidity. Many other curious experiments 

 with this simple instrument are related by Professor Wheatstone ; and they 

 will go to confirm the general conclusion, that the combination of the dis- 

 similar images furnished by the two eyes is a mental act, the resultant of 

 which, in the case of all objects that are near enough to be seen in different 

 perspective with the two eyes, is a mental image (referred to the visual 

 sense) possessing the attributes of solidity and projection. In regard to 

 distant objects, however, the difference in the images formed by the two <'\< s 

 is so slight that it cannot aid in the determination ; and hence it is, that 

 whilst we have no difficulty in distinguishing a picture, however well painted, 

 from a solid object, when placed near our eyes (since the idea which might 

 be suggested by the image formed on one eye will then be corrected by the 

 other), 1 we are very liable to be misled by a delineation in which the per- 



1 It is a remarkable illustration of this principle, that a photographic representa- 

 ti*n of a landscape, building, etc., when viewed with one eye at a moderate distance, 



