138 MICROMETRY AND BLOOD PREPARATIONS. 



power of the lens of the objective. If we were using a No. 4 ocular, 

 the magnifying power would be approximately 40. 



BLOOD PREPARATIONS. 



To obtain blood, except for blood cultures, use either a platino- 

 iridium hypodermic needle which can be sterilized in the flame, a 

 small lancet or a surgical needle with cutting edge. A steel pen with 

 one nib broken off or the glass needle of Wright may also be used. 

 To make a glass needle, pull straight apart a piece of capillary tubing 

 in a very small flame. Tap the fine point to break off the very delicate 

 extremity. Scarcely any pain attends the use of such a needle. In 

 puncturing either the tip of the finger or lobule of the ear a quick piano- 

 touch-like stroke should be used. The ear is preferable, as it is less 

 sensitive and there is less danger of infection. Before puncturing, the 

 skin should be cleaned with 70% alcohol and allowed to dry. It is 

 advisable to sterilize the needle before using it. 



The first drop of blocd which exudes should be taken up on the 

 paper of the Ta'lquist haemoglobinometer, using subsequent ones for 

 the blood pipettes and smears. If it is necessary to make a complete 

 blood examination, it is rather difficult to draw up the blood in the 

 pipettes, dilute it and then get material for fresh blood preparations 

 and films without undue squeezing, which is to be avoided. Of 

 course, fresh punctures can be made. Ordinarily, complete blood 

 examinations are not called for. It is only a white count or a differ- 

 ential count or an examination for malaria that is required. 



HEMOGLOBIN ESTIMATION. 



The most accurate instrument for this purpose is the Miescher 

 modification of the v. Fleischl haemoglobinometer. The glass wedge 

 for comparison with the diluted blood is the same in each instrument, 

 but by the use of a diluting pipette accurate dilutions are possible 

 in the Miescher. There are two cells provided one 12 millimeters 

 high, the other 15 millimeters; the idea of this being to enable one to 

 make separate comparisons and to select the central parts of the 

 glass-wedge scale, where comparison is more accurate than at the ends. 



If using a i to 200 dilution and the deeper cell, the reading of the 



