108 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and transparent objects in the Microscope; and when this erroneous 
rendering is supplemented by the artificial contrasts due to chemical 
intensification of the original negatives, or to the after-processes of 
copying and enlarging, it becomes of the first importance in cases where 
photographs are brought forward in illustration of special points, that 
either the original negatives, or reproductions of them as exact as possible, 
should be exhibited. When, as in many cases — notably by Dr. Van 
Heurck — enlarged photographs are brought forward without any precise 
description of the process of their production, and without the original 
photomicrographs for comparison, useful criticism is difficult if not 
impossible. All that can be said about them amounts to expressions of 
vague astonishment that the image should look so strong and so highly 
magnified. I think it would be advisable in all cases to distinguish 
between photomicrographs produced with the Microscope, and enlarged 
photographs from photomicrographs : the former can only be usefully 
criticized by one who is familiar with the object as seen in the Micro- 
scope ; the latter need other criteria whence their utility may be 
estimated, and, above all, they need the presence of the originals from 
which they were enlarged. 
(5) Microscopical Optics and Manipulation. 
The full Utilization of the Capacity of the Microscope, and 
means for obtaining the same.* — In a paper read before the American 
Society of Microscopists, Mr. E. Bausch said: — The cover-glass may 
truly be called a necessary evil, for while absolutely required in micro- 
scopical investigations, there is no adjunct to the Microscope that has 
been and is productive of so much evil, and has so retarded the utiliza- 
tion of benefits made possible by the advance in the construction of 
objectives. This fact was appreciated as early as 1837, when the angular 
apertures were what would now be considered extremely limited, and 
the appreciable effect of variations in thickness of cover-glass was not 
then nearly so pronounced as it is at the present time, even in modern 
objectives of a narrow angle. 
The accommodation for the different thicknesses was obtained by 
varying the distance between the systems of objectives, and has been 
followed with modifications in the mode of obtaining the necessary 
motion up to the present day. While open to some objection, it 
accomplishes the purpose quite satisfactorily, and must continue to be 
used until something better is suggested. 
One of the purposes of the homogeneous immersion is, as we know, 
the avoidance of the necessity of the cover-correction, in that the cover- 
glass, immersion fluid and front of objectives are to be one homogeneous 
mass, but even under these conditions, which in practice were found to 
be not constant, it has been found advisable to provide cover-correction 
to obtain the highest possible results. However, even should this not 
be found necessary in the development of improvements in this class of 
objectives, it must be remembered that the majority of objectives will 
always be dry, and especially so when such improvements, which we 
hope are still to be made, are accomplished. It is an unfortunate 
circumstance that with this class of objectives the influence of variation 
* Microscope, x. (1890) pp. 289-96. 
