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 unit in micrometry is the micron. This is usually written p 

 and is the i/iooo part of a millimeter. There are 1000 microns in a 

 millimeter. 



To standardize: For this purpose it is necessary to have a scale 

 of known measurements. The stage micrometers are usually ruled 

 in spaces of o.i and o.oi mm. The lines which are i/io of a milli- 

 meter apart are consequently separated by a distance of 100 microns; 

 those i/ 100 of a millimeter apart are separated by a distance of 10 

 microns. 



169 



