BLOOD PREPARATIONS 171 



30.6 microns. With the i/6-inch objective the 50 ocular spaces would cover about 

 three of the i/io millimeter (ioo/*) spaces of the stage micrometer. Then the 50 

 spaces would equal 300 microns and one space would equal 6 microns. 



The ruling of the slide of a Thoma-Zeiss haemocytometer will answer as well as 

 a stage micrometer. The small squares are 1/20 of a millimeter square, consequently 

 the distance between the lines bordering the small square is 1/20 millimeter or 50 

 microns. 



Now, if with the i /6-inch objective, the 50 lines on the ocular fill in the spaces of 

 six small squares, the length of such a space would be 50X6 = 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. 



The most accurate instrument for measuring is the filar micrometer. 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 1/2 inch, the focal distance would then be about i inch. 

 Dividing 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. 



