BLOOD PREPARATIONS I 99 



the distance between the lines bordering the small square is J^o mm. or 5 

 microns. 



Now, if with the ^-inch objective, the 50 lines on the ocular fill in the spaces of 

 six small squares, the length of such a space would be 50 X 6 = 300 microns. This 

 divided by 50 spaces would equal 6/*. 



Should there be 100 spaces on the ocular micrometer instead of 50, it would 

 only be necessary to divide the length in microns of the ruled surface of the stage 

 micrometer covered by the 100 lines of the ocular micrometer by 100. The quotient 

 would give the value in microns of each space of such an ocular micrometer. 



Filar Micrometer. The most accurate instrument for measuring is the filar mi- 

 crometer. These are expensive. Measurements can also be made with the camera 

 lucida, but it takes considerable time to make the adjustments necessary, so that it 

 is not convenient. With an ocular micrometer one can make measurements of 

 blood-cells, amoebae, etc., in a few seconds it only being necessary to slip in the 

 ocular micrometer. 



Rule for determining the magnifying power of microscopic lenses: Measure the 

 diameter of the lens of the objective in inches the approximate equivalent focal 

 distance is about twice the diameter. Dividing 10 by the equivalent focal distance 

 gives the magnifying power of the lens. This should be multiplied by the number 

 of times the ocular magnifies. Example: The diameter of the lens of the objective 

 was found to measure Y^ inch, the focal distance would then be about i inch. Divid- 

 ing 10 by i we have 10 as the magnifying power of the lens of the objective. If we 

 were using a No. 4 ocular, the magnifying power would be approximately forty. 



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. 



When using such surgical needles it is a good plan to sharpen the cutting edge on 

 a fine-grained whetstone. Afterward the needle should be sterilized by boiling. 

 Sterilization of a needle in the flame blunts the 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 lobe 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 blood which exudes should be taken up on the paper 

 of the Tallquist hsemoglobinometer, using subsequent ones for the blood 

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

 amination, it is rather difficult to draw up the blood in the pipettes, 



