340 THE NORTHERN MICROSCOPIST. 

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words, “to the venerable relics of the past zaive period of micro- 
scopical science, which was characterized by an unshaken conviction 
in the validity of the hypothesis that microscopical vision is in all 
essential respects the same thing as ordinary vision.” The “all 
round vision,” by virtue of which we are supposed, when looking at 
a minute cube, to see at the same time the top and all the sides 
(with the result of rounding off the corners and angles !), does 
not really exist, as can be shown by the application of the simplest 
laws of geometrical image formation. The different obliquities of 
the rays in an objective of wide aperture cannot give rise to any 
all-round vision, for in the Microscope there is no difference of 
perspective attendant upon oblique vision as with the naked eye. 
The difference of projection of successive layers which exists is 
ineffective, except in the case of binocular vision. This absence 
of perspective may be readily established by examining an object 
alternately by an axial and an oblique ray; it will be found that 
there is no shortening of the lines in the latter case, and no 
capacity in the Microscope, therefore, “for all-round vision.” 
Indeed if this theory were correct, microscopical vision, even of 
Plane objects and with very moderate apertures, would be entirely 
destroyed. 
Equally mistaken is the second branch of the view which I 
am considering, viz. that a wide aperture must, in the nature of 
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 Jayers only, and this 
is not effective unless we produce these images by different 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 ave 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 1-inch homogeneous-immersion objective of 
the wide aperture of 1'25 that “the sharpness and brilliancy of 

