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THE MICROSCOPE. 117 
or “cap” on the surface ; on agar the microbe forms a freely spread- 
ing white film with a rosy tint; on potato a thick, tallow-like, pink 
coat with sharply defined fringed contours. 4. That the first two 
species (a and b) are also met with commonly in the water of the river 
Dnieper, which flows through the town (vide Dr. Indovsky’s bacteri- 
oscopi¢ examination of the water, published in the Meditzinsksié Ob- 
ozrenié (Nos. 9 and 10, 1888, p. 975), while the peculiar pink micro- 
coccus seems to occur only in snow. 5. That, generally speaking, 
the microbes liquefying jelly in falling or recently fallen snow are 
met with invariably in far greater numbers than in snow which has 
been on the ground for some time ; this, in fact, very often contains 
only such bacteria as do not liquefy gelatine. 6. That the bacteria 
of snow originate partly from aqueous vapors which are transformed 
into snow, partly and chiefly from the air: that is, they are carried 
away by the snow-flakes on their passage through the atmosphere. 
MICROS COPY = 
Hints on Ocutar Micrometers.—At a recent meeting of the 
Troy Scientific Association, R. H. Ward, M. D., F. R. M.S., read 
the following interesting paper : 
Recent improvements in ocular scales suggest a consideration as 
to the qualities desirable in such rulings, the best plan for using 
them, and the degree of precision in measurement attainable thereby. 
The character of the lines themselves is an important feature. 
What is wanted is a true line without appreciable width, but still dis- 
tinct and conspicuous ; while the finest available lines have, when in 
use, a visible breadth that interferes with sharpness of comparison, 
at least at one end of the diameter to be measured. It is therefore a 
matter of skill and judgment to make, or to select, lines sufficiently 
plain to be readily seen, amidst confusion of details on the stage, 
and as fine as is compatible with that effect. Lines too fine to be 
used otherwise are rendered discernible by being filled in with 
graphite. 
It is obvious that lines intended as a rule for measuring, ought 
to be practically faultless in the regularity of their spacing, and for- 
tunately there is no difficulty in attaining such perfection. The best 
rulings hitherto known, and now available, have errors perceptible 
under the magnifying power of the microscope, which fact demands 
great caution in choosing a scale and in verifying or correcting its 
* Under this heading will be included descriptions of New Instruments, Micro- 
scopical Manipulations, Stains and Re-agents, Photomicrography, etc. 
