JULIUS RHEINBBHG ON STEREOSCOPIC EFFECT AND A 



being less distinct in certain directions than in? 

 i . ■: •-. . r varying in distinctness according to the position in 

 which it happens to lie in the held. 



How it has come about that these matters have been over- 

 looked is simple enough. As regards microscope images, 

 tention has been chiefly concentrated — and justly so — on the 

 perfection of the image of the object layer in true and perfect 

 focus iii the view plane, and for this particular plane other 

 editions prevail. It is the one layer which is free from 

 parallactic displacement, no matter which part of the objective- 

 may be used. It is also the layer for which the laws framed 

 from the study of the diffraction of light apply more particularly. 

 And <me of these laws is that the "diffraction discs," of which 

 the image in this plane is composed, vary inversely in size with 

 the aperture of the objective (or of the part of the objective) 

 utilised. Smaller discs mean greater resolving power so long 

 as the image magnification remains unaltered; therefore, for this 

 one plane, the larger the aperture of the objective employed, the 

 better the images, and the largest aperture available in binoculars- 

 is the half-objective. An instructive experiment consists in 

 viewing a Grayson band plate with a binocular microscope. The 

 effect of the semicircular shape of the half -objective may then be 

 shown by rotating the plate. When the rulings lie in the direction 

 of the straight edge of the half objective, a band with only about 

 half tin- number of lines per inch is resolved as when they lie 

 in the direction at right angles to this. 



Although within certain limits the same principles hold good 

 with respect to slightly out-of-focus layers,* the general feature 

 remains that diametrically opposite conditions apply, as regards 

 diminishing the size of the discs, when the layer of the object is 

 in true focus and when it is not. The one necessitates the 

 employment of parts of the objective aperture as large as possible ;. 

 the other requires them to be as small as possible. 



To which are we to give more weight, bearing in mind that 

 thi of st( reoscopic effect lies in viewing a great number 



of planes simultam ously ? Should we adapt our instrument for 

 the single plane in true focus, or for all those others seen at the 



' Th< ion of diffractive effects in planes out of focus is an intricate 



e, and it would go beyond the scope of this paper to enter into a 

 this subject. 



