ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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in thickness of cover-glasses is most apparent ; but since it is so, wo 
should, if possible, provide an agency which, eliminating the personal 
factor of efficiency, will give under all conditions, results closely equal 
to those under which the objectives were originally corrected. 
It is surprising to see how little attention is paid to this subject in 
the large majority of standard works on the Microscope. Almost all 
books give carefully prepared illustrations and descriptions showing the 
effect on the course of light by the interposition of the cover-glass, and 
after giving conclusive evidence of its disturbing influence, still, in a 
general way, say it is of little moment. Thus, in a German work of the 
highest standing, which has also been translated into the English 
language, is found the following utterance, freely translated : — 
In regard to modern Microscopes, which we have had opportunity 
to examine, we have not found the differences in thickness such as occur 
in commercial cover-glass, when, for instance, three to six are equal to 
a mm., have any noticeable influence on the microscopical image.” 
In another work of great popularity are found the following quota- 
tions : — “ That the effect of thickness of cover-glass has a great influence 
on the perfection of the microscopical image is beyond the slightest 
question, and certainly deserves the most careful attention of the 
optician as well as the observer, but whether the devices for its removal 
are of such great importance and so absolutely necessary as it is 
claimed, is another question.” 
“ On the other side, the difference in the cover-glass used in different 
directions for the most delicate preparations is hardly of any account. 
I, at least, possess, besides my individual preparations covered with 
glass of about 1/5 mm. thickness, a collection of objects which I obtained 
from London and Paris, in which there is such a slight difference of 
cover-glass thickness that I can observe them all with my objectives of 
powers from 2 to 1*3 mm. (equivalent to about 1/12 to 1/20 in.) 
without showing the slightest difference in optical qualities, and in the 
definition and clearness of the image under the same illumination, as I 
have convinced myself by careful comparative tests.” 
With such statements to guide the microscopists, it is not surprising 
that the subject should have received so little attention, and that any 
efforts to lead to improved methods of manipulating objectives should have 
almost completely failed because of a lack of the true understanding of 
their need and consequent failure to create interest. The belief is quite 
general that any time devoted to this subject is wasted, and might better 
be utilized in other directions. 1 hope to be able to show that this is 
entirely wrong, and may here say that while I may be considered an 
extremist in the other direction, my efforts emanate from the desire to 
put it in the power of every microscopist to obtain the highest possible 
results from his optical battery, and equal to those obtainable by the 
optician. 
When, in 1887, Prof. S. H. Gage addressed a circular letter to all 
opticians in the world inquiring for the dimensions of their standard 
tube-length, as well as for the thickness of cover-glass which they used 
as a standard in the correction of objectives, I looked forward to the 
result with considerable interest, as it would bring together data which 
it was impossible otherwise to obtain. 
