30 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



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 Pseudoscopic 

 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 ' 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. 



25. Now it is easily shown theoretically, that the Picture of any 

 projecting Object seen through the Microscope with only the right- 

 hand half of an Objective having an even moderate angle of aper- 

 ture, must differ sensibly from the picture of the same object 

 received through the Ze/^-hand of the same objective ; and further, 

 that the difference between such picture must increase with the 

 Angle of Aperture of the objective. This difference may be prac- 

 tically 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 object 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 scale the solid form of the object from which they 

 were taken. What is needed, therefore, to give the true Stereo- 

 scopic power to the Microscope, 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 stei'eoscopic but pseudoscopic ; the pro- 

 jecting parts being made to appear receding, and vice versa. The 

 reason of this is, that as the Microscope itself reverses the picture 

 (§ 20), 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 



