60 THE MICROSCOPE. April 
the ocularand apply a very little stiff balsam to the under- 
side of the diaphragm at two opposite points. Ravel out 
a very fine straight fibre of silk. 
Attach the fibres across the diaphragm and fasten to the 
balsam; then attach another fibre parallel to the first. 
You willthen have a good means of comparing the rela- 
tive sizes of objects. and by obtaining the distance value of 
the threads with a stage micrometer a valuable and accur- 
ate eye-piece micrometer may be obtained. 
Cleaning Diatoms.—The following is quoted from an 
old journal. Free the diatoms from water, as far as pos- 
sible, by decanting. Then cover witha liberal quantity 
of pure concentrated sulphuricacid. Then heat to boiling 
ina procelain evaporating dish, continue the heating until 
the white fumes of sulphuric acid begin to escape freely. 
Then while still boiling add saltpeter in bits the size of a 
pea and wait, after each addition, till the effervescense 
ceases before adding more; continue until the whole mass 
is white or light yellow. This will not usually consume 
more than five minutes. Then wash the cleaned diatoms 
with successive portions of distilled water as usual. 
The Heat Process.—- Many frail organisms with cilia or 
tentacles may be prepared for mounting with the cilia or 
tentacles expanded in the following manner: Have in 
readiness aslide with a well dried cell. Place the animals, 
to be mounted, while alive in a drop of water on the end of 
a plain slide.. See that the cilia or tentacles are expanded. 
Then quickly hold the slide with the animals over a stud- 
ent’s lamp from three to five seconds; remove and place 
the slide upona good heat conductor until cool; when cool 
pour the drops containing the object into the prepared 
cell; under a dissecting ’scope, arrange the animals and 
fill the cell with a preservative fluid, then cover and finish. 
Chloral hydrate, glycerine and glycerine jelly will be found 
to be good preservative fluids. Remember that the cilia 
and tentacles will be destroyed by the least rough hand- 
ling. A camel’s hair brush, delicately used, will be found 
most servicable in arranging the objects, 
