SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 113 
The first requisite is a very ¢hin section; the 
second, a very clean specimen, free from dirt or 
air-bubbles. To secure cleanliness, wash the leaf 
or stem or tuber perfectly clean before com- 
mencing to make sections, and place the sections 
in filtered water when they are made. Use a 
very sharp instrument, and cover the face of the 
stem, or whatever it may be, with water or alco- 
hol; the razor also should be wet before making 
each cut. 
Be extravagant in the number of sections cut, 
and select only the best. The selected sections 
will often require soaking for a considerable time 
in alcohol, to get rid of the air-bubbles. They are 
to be mounted in water, solution of acetate of 
potash, glycerine, or Canada balsam. A little 
experience will enable the operator to judge 
whether a section, examined under the microscope 
in water, requires a medium of higher refractive 
index in order to render it more transparent. 
Cells in which the cellulose envelope is com- 
paratively thin, as in the pith of exogenous 
stems, the epidermis of thin-leaved plants, etc., 
will show better in water. Thin longitudinal 
shavings of the wood of the coniferaee — pine, 
cedar, etc. — may be mounted in glycerine, after 
being soaked in alcohol to remove air from the 
cells. Of course water and glycerine may be 
mixed in any proportion which seems desirable, 
to secure a refractive index between the two; 
and it may be that the addition of chloride of 
cadmium, or chloral hydrate, to glycerine, for the 
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