PREPARATION OF FILMS. 159 
squeeze the nipple gently so as to force the blood and 
(subsequently) the water drop by drop into the cell. 
Interrupt the process occasionally and stir the contents 
of the cell with the metal handle of the measuring tube. 
Continue to add water until the cell is exactly full; 
this is the first step which presents the slightest diff- 
culty. Apply the coverglass; this must not enclose any 
air under it, nor cause any of the diluted blood to flow 
into the moat round the cell. 
The specimen is now ready for comparison with the 
standards. It is to be taken into a dark room and 
examined by the light of one of the candles. This is to 
be placed in front of the observer at a short distance 
from the specimen and standards, which must lie side 
by side. 
The viewing is best done by means of a camera 
tube which folds into the box containing the whole 
apparatus. It terminates in a diaphragm which is 
perforated by two small holes, one of which is to be 
placed over the centre of the specimen and the other 
over the centre of the standard. The latter is to be 
moved about until a disc is found which nearly or quite 
corresponds in colour with the diluted blood in the cell. 
If the correspondence is exact the process is at an end ; 
the number against the disc in question represents the 
percentage amount of hemoglobin. If there is no disc 
which exactly matches the specimen the latter is placed 
against the disc which is nearest to it but not so deep in 
colour. For example, if we found that the specimen 
was darker than the disc numbered 50, but paler than 
that numbered 60, then it would be placed opposite to 
50. Avslip of colourless glass is then applied over the 
specimen, and riders over the standard disc, until an 
exact match is obtained. If, in the case mentioned 
