SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 391 



same time ? I think you will find it rational — the more so as, 

 even in the single plane, we cannot secure equally good resolution 

 in all directions — that we should extend a good deal of considera- 

 tion to all those other layers ; and the best rule to be followed — 

 one which I believe Mi*. Taverner, from his experiments in 

 stereoscopic photo-micrography, has also arrived at — is : Use 

 circular stops (as in Fig. 8), having them just small enough to 

 secure a moderately fair image of the deepest layers which it is 

 required to see simultaneously with the others. In other words, 

 get the necessary depth of focus, but no more ; for in securing 

 more, the perfection of the image in other parts is being decreased. 

 Similar objects being exhibited under binocular microscopes in 

 which this rule has been followed, and under others in which 

 the two halves of the objective are left as usual, the improved 

 effect in the former is perceptible at a glance. 



The application of circular stops permits of another advantage 

 of a quite different description, in the cases where it is per- 

 missible to use them smaller in size than the semi-aperture of 

 the objective. We are enabled to a certain extent to correct 

 the inherent defect of super-magnification, in the direction of the 

 depth dimension, to which reference has been somewhat 

 frequently made in this paper ; for, reverting to our experiment 

 with the matches and unaided vision, we shall find that the 

 further the one match is held away from the other, the greater 

 is the difference of the apparent shift in position of either match 

 when viewed with left and right eyes alternately. In other words, 

 a plane appears further away the greater its parallactic dis- 

 placement. We have seen that, using any two parts of an 

 objective separately, parallactic displacements of any object 

 layer varies directly as the distance between the centres of the 

 parts used. In binoculars as used at present that distance is 

 always just equal to half the diameter of the objective * ; but 

 when we use small stops, there is nothing to prevent us increasing 

 or diminishing the distance, as Fig. 8 shows, and in this way 



* There is one exception to this, inasmuch as Abbe pointed out that 

 his form of binocular gives good stereoscopic effect when, instead of both 

 Eamsden circles being half covered up. one of them is left uncovered. In 

 the latter case the difference between the centres of the openings corre- 

 sponds to a quarter of the diameter of the objective, and the parallactic 

 displacements are only half as great. 



Journ. Q. M. C, Series II.— No. 59. 2S 



