OF MICROSCOPIC OBJECTS. 101 
Some of the large shells are destroyed by this method, 
but none that are fit for microscopic use. An oblique light 
shows these objects best. 
These are sometimes treated in the manner described in 
Chapter III., where the diatoms are spoken of, but many 
forms are liable to be injured by this severe process. 
It has been before stated that some of the zoophytes may 
be mounted dry, and others examined as opaque or trans- 
parent objects according to their substance. They are very 
interesting when examined in the trough whilst living, but 
to preserve many of them for future examination they must 
be mounted in some preservative medium. Sometimes this 
may be one of the liquids mentioned in Chapter V., but 
if possible they should be kept in balsam, as there is less 
danger of injury by accident to this kind of slide. This 
method of mounting presents some difficulties, but I think 
that all agree as to the trustworthiness of Dr. Golding 
Bird’s information on the subject, which appeared in the 
Microscopic Journal. Of this, space forbids me to give more 
than a condensed account, but I hope to omit nothing of 
moment to the reader for whom these pages are written. 
After stating that there are few who are not familiar 
with these exquisite forms, and have not regretted the great 
loss of beauty they sustain in drying, he informs us that 
from their so obstinately retaining air in the cells and tubes 
when dried, it is hardly practicable to get rid of it; and they 
also shrivel up very seriously in the process of drying. The 
following plan, however, he has found almost faultless in 
their preparation. 
To preserve them with extended tentacles, they should 
be plunged in cold fresh water, which kills them so quickly 
that these are not often retracted.* The specimens should 
be preserved in spirit until there is leisure to prepare them; 
* It has been stated that the best method of killing zoophytes is to 
drop alcohol, French brandy, or benzole into the salt water in which 
they are placed ; as this will cause no retraction of tentacles if it be 
done gradually. 
