ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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flanked on each side at the limit of the aperture by about half of an 
Abbe “ spectrum ” image. The more distant halves of the “ spectra ” 
were just outside the limit of the aperture, and could not be seen. 
Then, for the diaphragm with an opening 10 mm. in diam. at the 
back of the objective was substituted a diaphragm having an opening 
6 mm. in diam. The latter just covered both halves of the two flanking 
<£ spectra,” and left on each side of the central image a breadth of dark- 
ness corresponding with one portion of the aperture neglected in the Abbe 
theory. On replacing the eye-piece and again looking at the test plate, 
the closer lines could not be seen. Resolution failed, because under 
the conditions present the Abbe theory requires for resolution the 
admission, by the diaphragm at the back of the objective, of at least 
a part of one “ spectrum ” image in addition to the central image of the 
opening in the diaphragm of the substage condenser. Again the eye- 
piece w r as removed. The diaphragm of the substage condenser was 
turned so as to bring opening “ 3 ” into use. This change caused the 
central image seen at the back of the objective to increase in size until 
it filled the opening 6 mm. in diam. in the diaphragm at the back of 
the objective. On replacing the eye-piece and looking at the test plate 
once more, the closer lines were seen. Resolution returned as a result 
of the additional light from the larger opening on the diaphragm of the 
substage condenser reaching and utilising the portions of aperture 
which were previously dark under the conditions necessary to the Abbe 
theory. 
Dr. Mercer summarises the results of his experiments thus : — • 
(1) Diffracted rays leaving an object may be considered in the same 
category with other rays changed in direction by an object. 
(2) The diffraction phenomena seen in a projected image are essenti- 
ally the effect of changes in light above the objective due to a function 
of aperture, and not to changes below the objective due to diffraction of 
light in the plane of the object. 
(3) Diffraction in the plane of the object does, under some circum- 
stances, furnish light to certain parts of an aperture from which primary 
rays are absent, and thus enables aperture to more fully determine the 
character of the projected image, resulting in a more nearly truthful 
image, or, on the other hand, in false appearances. This is the gist of 
the Abbe phenomena of microscopic vision. 
(4) But such phenomena are not peculiar to microscopic vision, not- 
withstanding Prof. Abbe’s claim to the contrary. 
(5) With any positive lens similar and more brilliant results may 
be got by utilising corresponding pencils of primary rays, instead of 
isolated pencils of diffracted rays. 
(6) Still more trustworthy results may be got by using continuous 
apertures three-fourths (in diameter) full of primary rays instead of 
the isolated pencils of primary rays. 
(7) An advantage peculiar to using narrow cone illumination with 
an objective of wide aperture (the only illumination admissible in the 
Abbe theory) consists in giving, under suitable conditions, apj)roximately 
the acme of resolving power simultaneously in each of several diameters. 
Thus a circular aperture is approximately squared or made rectangular 
as to resolving power in several of its diameters simultaneously. 
