374 JULIU8 BHEINBERG ON STEREOSCOPIC EFFECT AND A 



bulffine the lens brings the object to a sharp focus on the retina, 

 helps us to judge of the distance. 



{[) Ocular parallax — i.e. the way in which, when we move 

 the position of the eye, the images of the objects nearer or 

 further away than the one we are regarding appear to shift their 

 position in the same or in the opposite direction. 



5 i The amount of haziness or clearness with which the objects 



are seen plays a certain role — e.g. objects a little distance away 



appear considerably further off to a short-sighted person when 



he removes his spectacles, because indistinctness of image is 



ciated with the idea of distance. 



(6) Colour effects, due to physiological as well as psycho- 

 logical causes — blue, for example, suggesting greater distance 

 than red. 



7 1 Shadow effects — the position of the shadows enables judg- 

 ment to be formed as to shape and position of the object. 



The further causes of stereoscopic vision, when we use both 

 eyes together, depend chiefly upon : — 



(1) The mental blending of the dissimilar pictures formed on 

 the retii ae of the two eyes, because of the different position of 

 the eyes. 



(2) The degree of convergence of the eyes, unconsciously 

 tired by the degree of tension on the muscles by which 



this convergence is effected. 



Coming now to the microscope, we find that of the causes in 

 the first category quite a number disappear. To begin with, the 

 important aid furnished by "distance perspective" has to be dis- 

 pensed with. In the microscope, areas of similar size at different 

 distances yield pictures of similar size. This is easily susceptible 

 of experiment. Take two millimetre scales on glass, place them 

 on the object-stage on top of one another, so that the ruled 

 surface is separated by the thickness of the glass (or, placing the 

 upper scale face downwards, they can be separated by any 



ivenient amount of cardboard or paper), and view them 



in the microscope, using a 1-in. or 2-in. objective. 



When the lines on the upper scale are in sharp focus, the divisions 



on the ; scale will practically superpose and correspond in 



e.* 



Bcient penetrating power to see both scales at once it is 

 an iiis diaphragm (Davis shutter) over the objective. 



