160 Transactions of the Society. 



things, impair definition on account of the increase, thereby pro- 

 duced, in the dissimilar images received through the several parts 

 of the objective. In support of this view, illustrations drawn 

 from stereoscopic vision are adduced, which admittedly does 

 depend upon the dissimilar images formed by the right and left 

 hand halves of the objective ; but, as Professor Abbe has shown, 

 the dissimilarity of images presented by an objective of wide 

 aperture is a dissimilarity in the projection of successive layers 

 only, and this is not effective unless we produce these images by 

 difi'erent portions of the aperture separately and conduct them to 

 different eyes, as in binocular Microscopes. The sole effect of the 

 wider aperture when the images are not so separated, is a reduction 

 in the depth of vision — to confine us to the vision of thinner objects, 

 not to impair the definition of what is seen when the objects are 

 within the range of penetration. 



If we pass to practical experience, we shall find that the 

 principles which theory establishes are amply confirmed. All 

 who have worked with wide-angled objectives cannot fail to have 

 recognized the great fact of modern practical optics, the perfection 

 of definition obtained with such glasses — a fact which has been 

 verified by such authorities as Mr. Dallinger, who, so long ago as 

 1878, stated of a new |-inch homogeneous-immersion objective of 

 the wide aperture of 1"25 that "the sharpness and brilliancy of 

 the definition which this lens yields is absolutely unsurpassed in 

 my experience." 



The question of the power of resolution supposed to be pos- 

 sessed by small apertures can also be brought to a very simple 

 practical test by those who believe in that view exhibiting here to 

 the appreciative assemblage which they would have around them, 

 say 75,000 lines to an inch resolved with the low apertures 

 referred to ! 



We have seen that on the one hand the depth of vision 

 decreases as the aperture is increased, and that on the other as the 

 objects become smaller and smaller the similarity of their images 

 increases with the increase in the aperture — the one representing a 

 disadvantage attendant upon large aperture and the other an 

 advantage — and bearing this in mind we are in a position to arrive 

 at a correct view of the relative value of objectives with large and 

 small apertures, which I take to be this : — 



Both kinds of objectives are necessary for investigations into 

 the structure of minute objects, and an observer to be fully 

 equipped, should provide himself with two objectives, one of 

 moderate and one of wide apertm'e. The former would be 

 used for the more general survey of the various parts of the 

 object, and the latter for the subsequent examination of its minute 

 structure. In searching, for instance, through a stratum of fluid 



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