26 Transactions of the Society. 



(3) Dissimilarity of the images of solid objects by different parts 

 of the aperture is solely difference of projection (orthogonal pro- 

 jection versus oblique projection — or one degree of obliquity by 

 axial pencils against an opposite obliquity by oblique pencils). 

 It relates therefore exclusively to the manner in which successive 

 layers are seen projected to the common ground plane (per- 

 pendicular to the axis of the Microscope) or to the percej)tion of 

 the depth, and not in any way to the delineation of the plane 

 layers themselves. The effectiveness of this dissimilarity for micro- 

 scopic vision is confined to the case of an actual separation of the 

 images by stereoscopic apparatus ; for if this dissimilarity should 

 be perceptible and the partial images not separated (viewed by 

 distinct eyes), the out-of-focus layers would appear confused, and 

 no vision of the depth could be possible, as explained just above. 

 We have, then, no advantage from the said dissimilarity. 



(4) Stereoscopic vision in the Microscope is entirely based on 

 the said dissimilarity of projection exhibited by the different 

 parallactic displacements of the images of successive layers on the 

 common ground plane of projection. There is no true perspective 

 difference of the images by different portions of the aperture, 

 because the microscopic image does not admit of a perspective 

 shortening of the lines, which are oblique to the direction of the 

 delineating pencils. 



