PREPARATION OF FILMS. I5Q 



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 diffi- 

 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 haemoglobin. 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. A slip 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 



