CHAPTER IV. 
MICROMETRY AND THE CAMERA LUCIDA. 
1. The Stage Micrometer.—Objects may be directly 
measured under the simple microscope by the use of 
dividers, whose spread is determined, under the lens, 
by means of a steel scale divided into fifths of a milli- 
meter. With higher powers, some special standard is 
required; and glass slides bearing fine lines ruled at 
intervals of .1 and .ot mm. are prepared for this purpose. 
These are known as stage micrometers. The unit in 
which the measurement of microscopic objects is com- 
monly expressed is .cof mm., known as the micro- 
millimeter or micron, and denoted by the Greek letter p. 
Obviously such a scale is the only absolute essential 
for micrometry or measurement with the microscope, 
and it would be possible to find the dimensions of an 
object by simply placing it on the ruled slide and reading 
off the number of spaces covered. This process is, how- 
ever, open to two objections. Accurately graduated 
stage micrometers, which are expensive, would be too 
subject to breakage to make their employment in this 
manner economical. Furthermore, on such a scale it is 
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