OPTICAL PRINCIPLES OF THE MICROSCOPE. 27 



with the small square in the centre, and the four sides sloping equally 

 away from it; whilst the combination of the lower pair, c, D (which are 

 identical with the upper, but are transferred to opposite sides), no less 

 vividly brings to the mind the visual conception of a receding Pyramid, 

 ;still with the small square in the centre, but the four sides sloping equally 

 towards it. 



31. Thus we see that by simply crossing the picture in the Stereos- 

 cope, so as to bring before each eye the picture taken for the other, a 

 ' conversion of relief is produced in the resulting solid image; the pro- 

 jecting parts being made to recede, and the receding parts brought into 

 relief. In like manner, when several objects are combined in the same 

 crossed pictures, their apparent relative distances are reversed; the remo- 

 ter being brought nearer, and the nearer carried backwards; so that (for 

 example) a Stereoscopic photograph representing a man standing in front 

 of a mass of ice, shall, by the crossing of the picture, make the figure 

 appear as if imbedded in the ice. A like conversion of relief may also be 

 made in the case of actual solid objects by the use of the Pseudoscope; 

 an instrument devised by Prof. Wheatstone, which has the effect of re- 

 versing the perspective projections of objects seen through it by the two 

 eyes respectively; so that the interior or a basin or jelly-mould is made to 

 appear as a projecting solid, while the exterior is made to appear hollow. 

 Hence it is now customary to speak of stereoscopic vision as that in which 

 the conception of the true natural relief of an object is called-up in the 

 mind, by the normal combination of the two perspective projections 

 formed of it by the right and left eyes respectively; whilst by pseudosco- 

 pic vision, we mean that 'conversion of relief which is produced by the 

 combination of two reversed perspective projections, whether these be 

 obtained directly from the object (as by the Pseudoscope), or from 

 * crossed ' pictures (as in the Stereoscope). It is by no means every solid 

 object, however, or every pair of stereoscopic pictures, which can become 

 the subject of this conversion. The degree of facility with which the 

 4 converted ' form can be apprehended by the Mind, appears to have great 

 influence on the readiness with which the change is produced. And 

 while there are some objects the interior of a plaster mask of a face, for 

 example which can always be 'converted* (or turned inside-out) at 

 once, there are others which resist such conversion with more or less of 

 persistence. 1 



32. Now it is easily shown theoretically, that the picture of any pro- 

 jecting object seen through the Microscope with only the rtgrM-hand 

 half of an objective having an even moderate angle of aperture, must 

 differ sensibly from the picture of the same object received through the 

 70/Y-hand of the same objective; and further, that the difference between 

 such pictures must increase with the angular aperture of the objective. 

 This difference may be practically made apparent by adapting a ' stop ' 

 to the objective, in such a manner as to cover either the right or the left 

 half of its aperture; and by then carefully tracing the outline of the ob- 

 ject as seen through each half. But it is more satisfactorily brought into 

 view by taking two Photographic pictures of the object, one through each 

 lateral half of the objective; for these pictures when properly paired in 

 the Stereoscope, give a magnified image in relief, bringing out on a large 



figures with the axis of the eyes brought into convergence upon a somewhat 

 nearer point, so that A is made to fall on B, and c on D. 



1 For a fuller discussion of this subject, see the Author's " Mental Physiology," 



168-170. 



