SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 115 
spherical starch-grains, and acicular crystals (raph- 
ides), enclosed in the large cells of the pith. 
The beginner cannot do better than to take a 
common potato to practise on for his first lesson 
in cutting thin sections. An extremely thin sec- 
tion, mounted in glycerine, is a beautiful object, 
but one rather difficult to photograph. 
DIATOMS. 
The silicious frustules of diatoms will always be 
favorite objects with microscopists, on account of 
the variety and beauty which they present as to 
form and markings, and because they serve as 
tests of the resolving power of objectives. They 
are especially well adapted for photo-micrography, 
and the beginner cannot do better than to select 
for his first attempt one of the least difficult 
test-diatoms with moderately high powers. Tii- 
ceratium favus (Plate XIII.), which is No. 1 of 
Moller’s “Probe-Platte,’ is a good one to com- 
mence with. 
A good half-inch or four-tenths-inch objective 
will be better than a higher power, as the slightly 
convex form of the frustule calls for penetrating 
power. The amplification may be from two hun- 
dred to five hundred diameters. 
Navicula lyra may be tried next, with a good 
one-fifth or one-sixth-inch objective, and an ampli- 
fication of six hundred to one thousand diameters, 
From this the student may go to Pleurosigma angu- 
latum, or P. quadratum, using an immersion one- 
tenth. 
