after staining are intended
for micrometric observations) it is almost essential to fix by exposure
to a uniform temperature of 115 deg. C., for twenty minutes. This is best
done in a carefully regulated hot-air oven.

Fixation may also be effected by immersing in some fixative fluid, such
as one of the following:

1. Absolute alcohol, for five to fifteen minutes.

{ equal parts, for five to thirty
2. Absolute alcohol, { minutes (e. g., for blood or
Ether, { milk).

3. Osmic acid, 1 per cent. aqueous solution, for thirty seconds.

4. Corrosive sublimate, saturated aqueous solution, for five minutes.

5. Corrosive sublimate (Lang), for five minutes. This solution is
prepared by dissolving:

Sodium chloride 0.75 gramme
Hydrarg. perchloride 12.00 grammes
Acetic acid 5.00 grammes
In distilled water 100.00 c.c.
Filter.

6. Gulland's solution, for five minutes. This solution is prepared by
mixing:

Absolute alcohol 25.0 c.c.
Ether 25.0 c.c.
Corrosive sublimate, 20 per cent. alcoholic solution 0.4 c.c.

7. Formalin 10 per cent. aqueous solution (= 4 per cent. aqueous
solution of formaldehyde since formalin is a 40 per cent. solution of
the gas in water).

Either of these methods of fixation coagulates the albuminous material
and ensures perfect adhesion of the film to the cover-slip.

_Clearing._--Wash the cover-slip thoroughly in running water and proceed
with the staining.

If the film has been prepared from broth, liquefied gelatine, or pus or
other morbid exudations, saturate the film after fixation with acetic
acid 2 per cent. and allow it to act for two minutes.

Wash with alcohol, then let the alcohol remain on the cover-slip for two
minutes. (This will "clear" the groundwork and give a much sharper and
cleaner film than would otherwise be obtained.)

If the film has been prepared from blood or bloodstained fluid, treat
with acetic acid 2 per cent. for two minutes after fixation. Wash with
water, dry, and proceed with the staining. (This will remove the
haemoglobin and facilitate examination.)

_Staining._--

1. Rest the cover-slip, film side uppermost, on the rubber mat.

2. By means of a drop-bottle, cover the film side of the cover-slip with
the selected stain, allow it t