PART II 

 STUDY OF THE BLOOD 



CHAPTER XIII 

 MICROMETRY AND BLOOD PREPARATIONS 



MlCROMETRY 



IN the examination of blood and faeces preparations, especially 

 when the identification of animal parasites is in question, there is noth- 

 ing that assists more than a knowledge of the measurements of the 

 object studied. The making of such measurements microscopically is 

 termed micrometry. 



Micrometry is also indispensable in bacteriology and cytodiagnosis 

 as well as in animal parasitology. 



The most practical way of making these measurements is with an ocular microme- 

 ter. These can be bought separately, or a glass disc (disc micrometer) with lines 

 ruled on it can be dropped into the ocular to rest on the diaphragm inside the ocular. 

 The ruled surface of this glass diaphragm should be placed downward. As was 

 stated in connection with the microscope, the image of the object is formed at the 

 level of the diaphragm rim inside the ocular, consequently the lines of the image cut 

 those of the lines ruled on the glass in the ocular. Once having standardized the 

 value of the spaces of the ocular micrometer for each different objective, all that is 

 necessary subsequently in measuring is to count the number of lines or spaces which 

 the image of the object fills and then, knowing the value of each space for that ob- 

 jective, to multiply the number of spaces by the value of a single space. 



The Micron. The unit in micrometry is the micron. This is usually 

 written ju and is the Ho 00 P art of a millimeter. There are 1000 microns 

 in a millimeter. 



To standardize: For this purpose it is necessary to have a scale of known measure- 

 ments. The stage micrometers are usually ruled in spaces of o.i and o.oi mm. The 

 lines which are Ho mm. apart are consequently separated by a distance of 100 

 microns; those Koo mm. apart are separated by a distance of 10 microns. 



Ocular Micrometer. The ocular micrometer is usually ruled with 

 50 or 100 lines or spaces, separated by longer lines into groups of 5 

 and 10. 



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