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



that of the Phantasmagoria, in which the alteration in the size of the images 

 on the screen suggests the notion of their approach or recession, although we 

 are quite sensible that the distance of the screen from our eyes remains con- 

 stantly the same. In the second experiment, on the other hand, the picture 

 whose perceived magnitude is undergoing diminution or enlargement in con- 

 sequence of increase or lessening of the angle of convergence, does not appear 

 either to approach or recede, and yet, when attentively regarded in different 

 fixed positions, it is felt to be at different distances. Hence, as Professor 

 Wheatstoue observes, convergence of the optic axes suggests fixed distance 

 to the mind, whilst variations of retinal magnitude suggest change of dis- 

 tance; and, however paradoxical it may seem, "we may perceive an object 

 approach or recede, without appearing to change its distance, and an object 

 to be at a different distance without appearing to approach or recede." 1 A 

 like alteration in apparent size is produced when two pairs of figures (such 

 as those given in Fig. 273), the effect of one of which is to suggest a pro- 

 jecting, and that of the other a receding form, are viewed at the same time 

 in the ordinary Stereoscope. For it will be observed that the relative size 

 of the parts which appear to project is reduced, whilst that of the apparently 

 receding parts is augmented ; as is particularly the case with the square 

 truncated end of the pyramid, which is estimated by most persons as from 

 one-third to one-half larger in each of its dimensions in the receding, than 

 it is in the projecting pyramid, notwithstanding that the actual sizes of the 

 squares in the two sets of figures are precisely the same. For supposing H I 

 (Fig. 274) to represent the real side of one of the small squares, which be- 

 comes the truncated end of the pyramid ; when this is brought forward by 

 the mind into the position K L, as the truncated top of a projecting pyramid,- 

 being seen under the visual angle H A I, its apparent size is reduced to F G ; 

 whilst, on the other hand, the very same square, carried back by the mind 

 to the distance D E, as when it forms the truncated end of the receding pyra- 

 mid, is mentally enlarged to the dimensions B c, the visual angle B A c being 

 the same as H A I. 



627. The large share which the Mind has in the interpretation of even 

 such visual impressions as seem to us necessarily to induce particular percep- 

 tions, is further shown by a very remarkable class of phenomena, termed by 

 Professor Wheatstone (their discoverer) Conversions of Relief. The simplest 

 example of this class is presented by the alteration in the visual product of 

 the same Stereoscopic pictures, when their positions are transposed. Thus 

 the very same diagrams, which as placed in the upper part of Fig. 273, 

 bring before the mind's eye the conception of a projecting pyramid, when 

 changed to the position which they occupy in the lower part of that figure, 

 call up the image of a receding pyramid. And a corresponding effect is 

 produced by the reversal of any other pair of Stereoscopic pictures; all that 

 should project being made to recede, all that should recede being made to 

 project, provided the converse has any meaning which the Mind can readily 

 appreciate. But the same effects may be produced, if the objects themselves 

 are looked at by an instrument devised by Professor Wheatstone, and termed 

 by him the Pseudoscope ; the optical effect of which is, to reverse the ordinary 

 visual relations between the near and distant parts of an object; the two con- 

 ditions described in the preceding paragraph being combined inversely, so 

 that as an object or part of an object is nearer the eye, its larger picture 

 on the retina is accompanied by a diminished convergence of the optic 



1 See Philos. Transact., 1852, pp. 2-5. The Author thinks it well to add, that he 

 has himself verified the above very curious results; which are scarcely less valuable 

 contribution* t<> tin- Physiology of Binocular vision, than those earlier attained by 

 the same eminent experimentalist. 



