214 THE ELEMENTS OF VEGETABLE HISTOLOGY 



to 0.1 millimeter. After the accuracy of these scales 

 is checked by comparison with a stage micrometer, 

 they may be used for the approximate measurement 

 of fairly large objects. The diameter of the micro- 

 scope field with different combinations of objectives 

 and oculars may be ascertained by placing an object 

 at the edge of the field, noting the scale reading, then 

 moving the object to the opposite edge of the field 

 and making a second scale reading. The difference 

 in readings will give the diameter of the field with a 

 given combination of lenses. 



MICROMEASUREMENTS 



Occasionally it is necessary to ascertain the dimen- 

 sions of cells and cell contents. This may be accom- 

 plished by several methods, but the accessories usually 

 employed for this purpose are the ocular micrometer 

 and the filar micrometer. Ocular micrometers, while 

 not as accurate as the filar types, are less expensive 

 and sufficiently accurate for all but the most critical 

 observations. 



Ocular Micrometer. — This apparatus consists of 

 a glass disk of proper size to fit within the ocular 

 mounting and having a graduated scale ruled upon 

 its surface. The number of scale divisions varies; 

 but, as a rule, the fifth and tenth lines are longer 

 than the others. As the value of these scale divisions 

 varies with different microscopes and combinations 

 of lenses, it is necessary to standardize the ruling by 

 comparing it with a stage micrometer scale. The 

 stage micrometer is a slide upon which is etched an 

 accurately ruled scale 1 or 2 millimeters in length, 

 each millimeter being divided into 100 parts. 



