e the 19 mm. squares for ordinary
cover-glass film preparations, and 38 by 19 mm. rectangles for blood
films and serial sections; both varieties must be of "No. 1" thickness,
which varies between 0.15 and 0.22 mm., that they may be available for
use with the high-power immersion lenses.

Cover-slips should be cleaned in the following manner:

1. Drop the cover-slips one by one into an enamelled iron pot or tall
glass beaker, containing a 10 per cent. solution of chromic acid.

2. Heat over a Bunsen flame and allow the acid to boil gently for twenty
minutes.

NOTE.--A few pieces of pipe-clay or pumice may be placed in
the beaker to prevent the "spurting" of the chromic acid.

3. Turn the cover-slips out into a flat glass dish and wash in running
water under the tap until all trace of yellow colour has disappeared.
During the washing keep the cover-slips in motion by imparting a
rotatory movement to the dish.

4. Wash in distilled water in a similar manner.

5. Wash in rectified spirit.

6. Transfer the cover-slips, by means of a pair of clean forceps,
previously heated in the Bunsen flame to destroy any trace of grease, to
a small beaker of absolute alcohol.

Drain off the alcohol and transfer the cover-slips, by means of the
forceps, to a wide-mouthed glass pot, containing absolute alcohol, in
which they are to be stored, and stopper tightly.

NOTE.--After once being placed in the chromic acid, the
cover-slips must on no account be touched by the fingers.

~Used Slides and Cover-slips.~--Used slides with the mounted cover-slip
preparations, and cover-slips used for hanging-drop mounts, should, when
discarded, be thrown into a pot containing a 2 per cent. solution of
lysol.

After immersion therein for a week or so, even the cover-slips mounted
with Canada balsam can be readily detached from their slides.

_Slides._--

1. Wash the slides thoroughly in running water.

2. Boil the slides in water to which "sapon" has been added, for half an
hour.

3. Rinse thoroughly in cold water.

4. Dry and polish with a dry cloth.

_Cover-slips._--

1. Wash the cover-slips thoroughly in running water.

2. Boil the cover-slips in 10 per cent. solution of chromic acid, as for
new cover-slips.

3. Wash thoroughly in running water.

4. Pick out those cover-slips which show much adherent dirty matter, and
rub them between thumb and forefinger under the water tap. The dirt
usually rubs of