374 C, H, GOLDING-BIRD. 
note of it can be conveniently made upon the cardboard that 
occupies the place of the labels—the instrument can on any 
future occasion be raised to the required temperature in a few 
minutes. To graduate it place upon the copper disc a glass 
cover (five eighths or three quarters of an inch square), care 
being taken that it does not touch the coils of copper wire, 
and on the centre of the cover a small piece of cacao butter. 
The copper tongue, the large coils of copper wire, and the 
first two small coils, should each have upon them a small 
fragment of solid paraffin, the melting point cf which is much 
higher than that of cacao butter, and the top of the flame of 
a spirit lamp should play upon the iron wire just beyond 
the copper coils, or may, according to circumstances, 
include the first two or three. In the instrument from 
which this description is taken it is found that when this is 
done, and the stage warmed sufficiently to allow the cacao 
butter just to dissolve, that the paraffin on the small coils 
has completely melted, that on the large coils is just sticky, 
and that on the copper tongue unaffected. The spirit-lamp 
may now be moved a little further off if it be thought neces- 
sary, the guide being if the paraffin on the large coils shows 
a disposition to melt, all that is now required being to 
ensure sufficient heat being conveyed along the combined 
copper and iron wires to make up for that lost by the stage 
from conduction or radiation. Where only copper wire 1s 
used it is hard to prevent the heat rising too rapidly, and 
where iron alone it is difficult to prevent the reverse; but 
where the two are combined, and the flame applied to the 
iron, the copper seems to make up that deficiency in conduc- 
tion in the iron that renders it unfit to be used alone. It is, 
of course, evident that each instrument must be tested, for 
though the positions and conditions of the paraffin given 
have been noted from a warm stage, constructed as here 
described, yet variations will be met with according as the 
coils are more or less closely wound. ‘The way in which the 
cacoa butter melts should be observed also ; if a second piece 
be dropped upon the glass cover, and melt ¢mmedvately, the 
stage has been overheated; but if it gradually and delibe- 
rately subside in the course of a few seconds the required 
temperature may be fairly assumed to have been obtained. 
The longest time during which the same white blood-cor- 
puscle was watched on this stage was an hour and an 
half. The spirit-lamp was moved but once, and that because 
the middle piece of paraffin showed signs of liquefying, while 
at the end of that time there seemed to be no reason why the 
same observation should not, as far as the stage was con- 
