( 66 ) 
Already before t) I called attention to the fact, that preparations, 
colored after the moist way, are liable to lose their pigment through 
decoloring agents. In the counting-preparations also, this difficulty 
is met with, and, as Dr. F. HY. Henewerrn points out 2), a slight 
acid reaction of the preparations or of the Canada balsam ean already 
call forth a rather prompt decoloration. By using a somewhat con- 
centrated, neutrally reacting solution of xylol-Canada balsam, the 
decoloring can be sufficiently prevented during the time of the counting. 
I have now found that this decoloration should be attributed 
to the detaching from the cover-glass of the whole preparation, 
which the non-removed pigment has rendered consistent; the xylol- 
Canada balsam, beginning at the periphery of the cover-glass, enters 
between it and the preparation, and, in so doing detaches the latter 
from the glass. Hence, viscous Canada balsam has a. less prejudicial 
effect than a thinly liquid solution; acids too, further this softening- 
process, so that hereby the influence on the decoloring of the prepa- 
rations is sufficiently explained. The decoloration can be wholly 
prevented by making the substance adhere more firmly to the 
cover-glass by means of some viscous matter; I commonly use 
therefore a clarified 4—5 °/) solution of gelatin in water. Before 
using, tbe solution is melted and a very small loopful of the 
liquid gelatin is deposed on the cover-glass; the colored bacteria- 
emulsion is mixed on the cover-glass with the gelatin-solution 
and spread over the glass. After drying the preparation is not 
flamed, but directly enclosed in xylol-Canada balsam; the consistency 
or the reaction of the balsam has no more any influence: the prepa- 
rations remain colored for weeks. 
A second modification refers to the choice of the optic fields 
which are to be counted. To this end are beforehand marked on 
a scheme of millimeter paper the places of the preparation to 
be counted; the centre of the cover-glass serves as fixed point. The 
optie fields (they are numbered from 1 to 50) are chosen in sucha 
succession that they may casily be reached one after another by 
means of a movable object-stage, whilst the respective distances of 
the optic fields are taken in such a way that for example, 1 c.m. 
on the scheme corresponds with 1 m.m. displacing of the stage. 
1) ArpX. Krein, Hine einfache Methode zur Sporenfärbung. Centralbltt. für Bact. 
und Par., le Abth., 1899, Bd XXV, S. 376. 
2) KF. H. Henewerrm, De mieroscopische telmethode der bacteriën van ALEX, KLEIN 
en eenige van hare toepassingen. Diss. Amsterdam 1900. Die mikroskopische Zählungs- 
methode der Bacterién von Arex. KrLeiN und einige Anwendungen derselben. Archiv 
fiir Hygiene. Bd XXXIX, 1901, 8. 321—389, 
