CARPENTER'S I'. ABBE'S VIEW OF STEREOSCOPIC VISION 93 



perspective projections, whether these be obtained directly from the 

 object (as by the pseudoecope) or from 'crossed' pictures (as in the 

 stereoscope). It is by no means every solid object, however, or everv 

 pair of stereoscopic pictures which can become the subject of this 

 conversion. Tlie decree of facility with which the * converted ' form 

 can be apprehended by the mind appears to have great influence on 

 the readiness with which the change i* produced. And while there 

 are some objects the interior of a plaster mask of a face, for ex- 

 ample 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 



Xo\v it is easily shown theoretically that the picture of any 

 projecting object seen through the microscope with only the riyht- 

 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 left 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 then by 

 carefully tracing the outline of the object as seen through each half. 

 But it is more satisfactorily brought into view by taking two photo- 

 graphic pictures of the object, one through each lateral half of the 

 objective ; for these pictures when properly paired in the stereo- 

 scope ii'ive a magnified image in relief, bringing out on a large scale 

 the solid form of the object from which they were taken. What is 

 needed, therefore, to give the true stereoscopic power to the micro- 

 scope is a means of so bisecting the cone of rays transmitted by the 

 objective that of its two lateral halves one shall be transmitted to 

 the right and the other to the left eye. If, however, the image thus 

 formed by the right half of the objective of a compound microscope 

 were seen by the right eye, and that formed by the left half were 

 seen by the left eye, the resultant conception would be not stereo- 

 scopic but pseudoscopic, the projecting parts being made to appear 

 receding, and vice versa. The reason of this is, that as the microscope 

 itself reverses the picture, the rays proceeding through the right and 

 the left hand halves of the objective must be made to cross to the 

 left and the right eyes respectively, in order to correspond with the 

 direct view of the object from the two sides ; for if this second 

 reversal does not take place, the effect of the first reversal of the 

 images produced by the microscope exactly corresponds with that 

 produced by the ' crossing ' of the pictures in the stereoscope, or by 

 that reversal of the two perspective projections formed direct from 

 the object, which is effected by the pseudoscope. It was from a 

 want of due appreciation of this principle (the truth of which can 

 now be practically demonstrated) that the earlier attempts at pro- 

 ducing a stereoscopic binocular microscope tended rather to produce 

 a ' pseudoscopic conversion ' of the objects viewed by it than to 

 represent them in this true relief. 



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

 S 1CH-170. 



