CHAPTER IV. 

 MICROMETRY AND THE CAMERA LUCmA. 



I. 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 .i and .01 mm. are prepared for this purpose. 

 These are known as stage micrometers. The imit in 

 which the measurement of microscopic objects is com- 

 monly expressed is .001 mm., known as the micro- 

 millimeter or micron, and denoted by the Greek letter [t. 



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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