758 E. A. MINCHIN. 
treatment, except that the preparation is sealed up to avoid 
evaporation of the fluid medium, and in this way it is possible 
to obtain very perfect temporary preparations, which can be 
kept unaltered for at least some months. I made use, there- 
fore, of this method for trypanosomes, but on account of the 
medium, blood, in which they live, special precautions were 
necessary in the application of it. 
My first method was to take an ordinary microscope slide 
with a small depression hollowed out in the centre, and to 
cement on to this a ground-glass ring, surrounding the hollow. 
The upper edge of the ring was then painted with vaseline, 
and a drop or two of osmic acid solution (4 per cent.) placed 
in the hollow of the slide. Now a coverslip was taken and 
a drop of fresh blood placed in the middle of it and spread 
out with a clean glass rod, not, however, smeared out into a 
thin layer. It was then placed with the blood downwards on 
the ground-glass ring over the osmic acid, and the coverslip 
pressed down all round on the vaseline. All this manipula- 
tion was done with the greatest possible rapidity, in order to 
avoid evaporation of the blood as much as possible. Thus 
the blood in a hanging drop is exposed to the vapour of 
strong osmic acid in an air-tight chamber; it is, of course, 
important that the blood should not come into contact with 
the osmic acid solution. It is now possible to examine the 
trypanosomes without further treatment; but owing to the 
thickness of the preparation, difficulties arise in the illumina- 
tion of the object, since it is not possible to focus the sub- 
stage condenser properly for the use of the highest powers. 
I was able, however, to draw the trypanosomes in outline, at 
a magnification of 3000 (figs. 1, 2), but could not make out 
minute details. In order to do this a further manipulation 
was necessary. 
After the coverslip with the blood had been exposed to the 
osmic vapour for a certain time, it was picked off the glass 
ving and at once placed with the blood downwards on an 
ordinary clean side. From the glass ring the coverslip takes 
with it a ring of vaseline, which when pressed down on the 
