T o 



SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 37 



desire to draw attention is that the general configuration of a 

 section across any such disc is the same as that of the portion of 

 the aperture of the objective utilised. Thus, if we use the whole 

 objective the diffusion discs are circular ; if we use half of the 

 objective only the discs are half-moon shaped ; if we placed a 

 triangular stop over the objectives the diffusion discs would 

 become triangular (see Figs. 2 and 3). The bearing of this on 

 the subject of stereoscopic microscope images has, so far as I can 

 find, hitherto altogether escaped attention. We shall see its 

 importance a little later on. 



We must now digress a little to recall some of the causes of 

 our impressions of solidity and plasticity when we view objects 

 with the naked eye, as this will afford some basis for comparison 

 as regards similar effects in the microscope. Optical effects 

 alone do not account for our impression of solid form, unconscious 

 judgment and experience playing a role at least equally im- 

 portant. For our present purpose it will be convenient to 

 distinguish between : — ■ 



1. Causes which operate even if only one eye is used. 



2. Additional causes coming into play because of the use of 

 both eyes. 



To the former category belong in the first place : — 



(1) Perspective effects due to the fact that the angle at which 

 objects or areas of similar size subtend the eye alters with their 

 position, so that differently sized pictures are formed on the 

 retina. For our purpose it will be found necessary to sharply 

 distinguish between three kinds of such perspective effects, viz. : — 



(«) Those when any area is moved backwards or forwards 

 — i.e. parallel to the direction of the optic axis of the 

 eye. Let us call this " distance perspective." 



(b) When an area is moved sideways — i.e. at right angles 



to the direction of the optic axis of the eye. Let us 

 call this "position in field of view perspective." 



(c) When an area is inclined or rotated with respect to the 



optic axis of the eye (or if the eye itself be rotated). 

 Let us call this " inclination perspective." 



(2) Comparison with other objects of known form and position, 

 or comparison with similar objects in a different position. 



(3) The accommodation of the eye — the unconscious degree 

 of tension or relaxation of the muscles, which by flattening or 



