SUMMARY OF CURRENT RESEARCHES RELATING TO 
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scopic vision by diffracted rays originating in the object. Can an image 
of an opening, or of a source of light formed in the Microscope-tube 
between the objective and the field lens of the eye-piece by diffracted 
rays, have any influence on the primary ray-image of microscopic vision 
formed between the two lenses of the eye-piece ? Two widely separated 
images cannot be seen through the Microscope at the same time. In 
other words, such images cannot be united to form a joint visual 
picture. 
Dr. Mercer thinks that the Abbe “spectra” are sometimes present as an 
accident of Microscope projection, and sometimes they are not. 
Why do “ spectra ” when present appear to be of such importance ? 
The “ spectra ” are so placed in a plane at the back of the objective that 
when a slotted diaphragm in their plane uncovers the “ spectra,” the 
same diaphragm uncovers simultaneously certain emitting points of the 
projecting lens. If now one of the slots of such a diaphragm be covered, 
one of the “ spectra ” disappears, and a corresponding change occurs in 
the projected image. But this change is due to the loss of the slot and 
corresponding emitting point of the projecting lens, and not to the loss 
of one of the “ spectra ” ; because if the “ spectra ” were absent in full cone 
illumination , the covering the slot in the diaphragm would produce the same 
change in the projected image. 
When the axial illuminating pencil is narrow and the Abbe t: spectra ” 
are separated by well-marked intervals of darkness, the Abbe theory 
ignores the emitting surface of the objective corresponding with the 
intervals of darkness. In harmony with this partial neglect of aperture, 
resolution in the Abbe theory may be said to increase by jumps. So 
long as a central image of the source of light is to be seen at the back 
of the objective, resolution is not present. The aperture may be in- 
creased without change in the contraction of the diffraction pattern and 
in accompanying resolution, so long as the central image alone is to 
be seen at the back of the objective. But the moment the increase in 
aperture is sufficient to uncover or admit one flanking “ spectrum ” image, 
resolution is present. With greater aperture no improvement is to be 
seen until another “ spectrum ” image is uncovered or admitted. 
On the other hand, with full cone illumination, resolution increases 
continuously, and not by jumps or by periodic accessions. The portions 
of aperture neglected in the Abbe theory are effective in full cone illu- 
mination. They contribute in proportion to their breadth, radially from 
the principal axis, to the contraction of diffraction patterns. And thus 
they may resolve additional finer details (experiment 14) in an object, 
or increase the distinctnessyff the resolution of details already resolved. 
Experiment 14. — A Microscope was arranged to exhibit the lines 
shown in photos 10 and 35. For the optical part of a Powell and 
Lealand substage condenser was substituted a Powell and Lealand 1-in. 
objective. A Powell and Lealand 3-in. objective and a “ 10 compensat- 
ing ” eye-piece were used. A diaphragm with an opening 10 mm. in 
diam. was placed at the back of the objective. The revolving diaphragm 
of the substage condenser was turned so as to bring opening “ 1 ” into 
use. The closer lines of the test plate were resolved. On removing 
the eye-piece and looking at the back of the objective, a central image 
of the opening in the diaphragm of the substage condenser was seen, 
