ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
707 
scratch is made on the under surface of a cover- glass and on the upper 
surface of a slide. The fluid to be examined having been placed between 
the two glasses the scratches are brought the one over the other. r J he 
cubical contents of the fluid cylinder are then calculated by first 
ascertaining the diameter of the field with the system in use. This is 
done by drawing the apparent size of the field at the stage level, and 
then measuring it with (say) a pair of compasses. This diameter, 
divided by the magnification, gives the real size. Then the square of 
the real diameter multiplied by *7854 gives the area ( m ) required. The 
distance ( h ) between the two glasses or the height of the cylinder is then 
ascertained by careful and repeated focusing. Then m h is the solidity 
of the cylinder. The number of the corpuscles or bodies in the field 
Are then carefully and repeatedly counted and an average struck (jp). 
With these data the number of corpuscles in a cubic centimetre is easily 
calculated. An ocular micrometer divided into squares facilitates the 
counting. In examining blood this fluid should be mixed with an equal 
bulk of 0*75 per cent, salt solution; if this 
procedure be adopted it will be necessary to 
multiply p by 2. 
Counting-Plate forj Petri’s Capsules.* — 
Herr Miiller-Unkel has produced a counting- 
plate of black highly polished glass on which 
Are marked four concentric circles and sixteen 
radii. This form is more convenient than 
the older apparatus used for counting. 
Writing on Metal and Glass.f — Herr 
H. Schobel recommends the following inks 
for marking slides, bottles, vessels, &c. : — 
Silicate of soda 1-2 parts, fluid Chinese ink 
1 part, is suitable for slides ; while for metal vessels he prefers silicate 
of soda, Chinese white, 3-4 parts of each, sulphate of baryta 1 part. The 
mixtures should be kept well corked and shaken before being used. 
Demonstrating Bacillus coli communis in Water.J — The question 
whether water which contains Bacillus coli communis is fit for drinking, 
is, says Dr. Ed. von Freudenreich, an important one. Recent investi- 
gations have shown that the microbe occurs in almost every kind of 
•drinking-water, and the demonstration of its presence largely depends 
on the volume of water used in the test. 
Thus, by Vincent’s method, wherein about 100 ccm. are used (water 
90 ccm., 20 per cent, pepton solution 10 ccm., 7 per cent, carbolic acid 
1 ccm., and incubation at 42°) the presence of B. coli can almost always be 
demonstrated, though if 1 ccm. only be used it may not be detected. Atten- 
tion is called to the facts that — (1) In chemically bad water (i. e. water 
containing too much organic matter) wherein j bacteria are numerous, 
Bacillus coli communis is present in abundance. (2) In water containing 
few bacteria and chemically good, B. coli is present in small numbers 
■only. (3) Very often this organism is absent altogether in water which 
* Zeitschr. f. Angewandte Mikroskopie, i. (1895) p. 173 (1 fig.). 
f Tom. cit., p. 183. 
X Centralbl. f. Bakteriol. u. Parasitenk., l te Abt., xviii. (1895) pp. 102-5. 
3 A 2 
Fig. 124. 
