The President's Address. By E. M. Nelson 
153 
to the Abbe theory, Dr. Mercer says, “ Eesolution in the Abbe theory 
may be said to increase by jumps. So long as the central image of 
the source of light alone is to be seen at the back of the objective 
resolution is not present. The aperture may be increased without 
change in the contraction of the diffraction pattern, and in accompany- 
ing 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 increase in aperture no improvement in the 
picture as to the contraction of the diffraction pattern, and as to ac- 
companying resolution, is to be seen until another spectrum image is 
uncovered or admitted. On the other hand, with full cone illumina- 
tion, resolution increases continuously, and not by jumps or by peri- 
odic accessions.” With regard to the use of oblique light the author 
says, “ Photos 2, 3, and 4 are a pictorial warning for a second time, 
now a warning against the use of oblique illumination in ordinary 
work as a means of increasing, or of attempting to exhaust, the resol- 
ving power of the Microscope. At the same time it becomes evident 
that every substage should be provided with a means by which its 
condenser may be accurately centered, and that every student using 
the Microscope should be familiar with a method of centering his sub- 
stage condenser. These general rules should be accompanied by 
another : the substage condenser should be as well made as the best 
objective, and should be used with an ever present appreciation of its 
power to improve or injure the picture of microscopic vision.” 
It is plain, therefore, that American Microscopy as enunciated by 
the President of its most influential Society has rebelled against the 
Abbe theory. Let me recommend all interested in this subject to 
read this important paper. 
Apparatus . — There is a dissecting Microscope stand with a very 
well arranged polariscope described in the Journal ;* it is an instrument 
that will commend itself especially to petrologists. At the March 
meeting, Prof. Wright exhibited his adaptation of a ghost micrometer 
for the purpose of counting blood-corpuscles ; f this admirable plan 
will no doubt greatly assist those who have this kind of computations 
to make. The ghost micrometer for linear measurements was the inven- 
tion of Dr. Goring ; J it has however been reinvented several times, § 
but this adaptation of Prof. Wright’s is original. We next have a 
new mechanical stage with an extended transverse movement, j] 
And then there are six kinds of photomicrographic apparatus, of 
which two are simple, one for daylight, one for lamp, one for electric, 
and one for limelight illumination. 
Passing over the simpler forms of apparatus which do not call for 
any special notice, we come to Mr. J. E. Barnard’s for electric illumi- 
* Journal R.M.S., 1897, p. 163. f Tom. cit., p. 215. 
1 ‘ Micrographia ’ (Pritchard and Goring), 1837, p. 51. 
§ M.M.J., ix. (1873) p. 2. || Journal R.M.S., 1897, p. 185. 
