108 PHOTO-MICROGRAPHS. 
into details with reference to the preparation 
of objects, even if the technique were com- 
pletely worked out, which is far from being the 
case. 
The general statement may be made, however, 
that objects which, in water, are not sufficiently trans- 
parent for photography, should be mounted in media 
having a higher refractive index, of which the most 
useful are glycerine and Canada balsam. 
Much valuable information with reference to 
the mounting of objects in these media can be 
obtained from the various works on the microscope, 
and especially from the files of microscopical 
periodicals. The best of these is undoubtedly 
the “Journal of the Royal Microscopical Society ” 
(London), which is published bi-monthly. 
Some pollen-grains swell up and the mem- 
branous envelope is ruptured when they are 
immersed in water. For this reason, as well as 
for that already given, glycerine is commonly 
a more suitable fluid in which to mount them. 
When first placed in glycerine, the cell wall 
becomes collapsed from exosmosis of the watery 
contents; but after a time the natural form is 
recovered by endosmosis, and the fluid within 
and without is of the same density. 
To prevent the trouble arising from the pres- 
ence of air-bubbles, which are apt to adhere 
tenaciously to the pollen-grains, it is best to 
immerse them first in alcohol, as recommended 
for the spores of fungi. 
A drop of alcohol is placed in the centre of 
