SOME THOUGHTS ABOUT MOUNTING. 105 
sparing assistance given to the Commission by the Egyptian 
Government. The decision to proceed to Calcutta instead of 
Bombay was taken because the Commission learned that the 
cholera epidemic in the latter port had ceased. 
SOME THOUGHTS ABOUT MOUNTING. 
By C. Henry KaAIn. 
HE attention of working microscopists has been more earnestly 
directed of late to the matter of mounting media than ever 
before, and it is becoming more and more manifest that we have 
much yet to learn in this line. For many years microscopists were 
content to embalm everything in balsam; but such is no longer 
the case, for while it is still one of the most reliable mediums for 
many purposes, yet the microscopical student is learning to select 
a medium with special reference to the characteristics of the object 
to be displayed. Those who have examined the recent slides of 
diatoms mounted in phosphorus by Moller, or those mounted by 
Prof. H. L. Smith in his new medium, cannot fail to appreciate the 
fact that the selection of a proper medium is an important con- 
sideration as regards the revealing of structure. 
The writer has two specimens of /stimia enervis mounted while 
fresh, z. e., without being cleaned in acids—one in Deane’s medium 
and the other in balsam. ‘The first exhibits finely the endochrome 
and membranous envelope of the diatom, but the silicious frustule 
is scarcely visible. In the second the envelope is absolutely in- 
visible and only the silicious frustule can be seen. 
Mr. Stephenson has shown that this revealing of structure is due 
to the use of a medium having a refractive index different from 
that of the object, and that the measure of visibility of minute 
structures is equal to the difference between the refractive indices 
of the object and the medium in which it is mounted. This being 
the case, it becomes a matter of serious moment to determine what 
medium is best adapted to a specific purpose ; for not only must 
we keep in mind the proper displaying of structure, but also the 
fact that it is almost if not quite as important to preserve unchanged 
the characteristics of an object. One is likely to be suspicious of 
new methods, too; for it is not gratifying to a microscopist to look 
over his cabinet and find that mounts prepared in a certain way 
have become valueless in a few months. Hence, for valuable 
