THE STRUCTURE OF TRYPANOSOMA LEWISI. 761 
Before proceeding to describe in detail the results of the 
fixatives and stains used by me, I will say a few words about 
the methods of applying them. I began with the ordinary 
method of making smears on slides; this is quite suitable for 
smears that are to be dried off or fixed with osmic acid 
vapour, for which the procedure adopted was the following: 
a glass tube is taken, of suitable size and calibre, and pro- 
vided with a tightly fitting cork or stopper; at the bottom of 
the vessel are put twenty drops of 4 per cent. osmic acid 
solution with one drop of glacial acetic; when the smear has 
been made, the slide bearing it is placed with as little delay 
as possible into the tube containing the osmic acid and 
corked up. It is advisable to take precautions against any 
part of the blood-smear coming into contact with the osmic 
acid solution, if one wishes to be economical in the use of 
this most expensive re-agent. An exposure of thirty to forty- 
five seconds to the osmic vapour is sufficient; the fixation is 
practically instantaneous. Slides fixed with osmic vapour in 
this way may be (1) dried off, then fixed with absolute alcohol 
or methyl alcohol in the ordinary way; or (2) placed at once 
without drying into absolute alcohol, stained as desired and 
then dried off; or (3) fixed, stained, and mounted in Canada- 
balsam without ever having been dried. 
I soon, however, abandoned the use of slides for smears for 
various reasons. ‘In the first place it is a clumsy method for 
rapid manipulation, and however anxious one may be to 
shorten the exposure to the free air, it is very difficult to avoid 
a certain amount of drying taking place in such large smears, 
either at the edge or in thin places. In the second place, 
slide smears do not give good results when the fresh wet 
smear has to be fixed by sudden immersion in a liquid, which 
it almost necessarily enters with a dive, so to speak, with one 
end foremost; in such cases it is common to find all the blood- 
corpuscles drawn out in an extraordinary manner, and all the 
trypanosomes stretched out straight in one direction, indicat- 
ing the line of the dive into the fixative. In the third place 
I may point out that since the best immersion objectives are 
VOL. 53, PART 4.—NEW SERIES. 54. 
