MICROMETRY MICROMETRIC MICROSCOPES 143 



According to Rogers 1 it is impossible to obtain true values 

 with certainty closer than 0.2 /*, this value being, as we have 

 already seen, the practical limit of the resolving power of the 

 compound microscope (see page 15). But when a series of 

 measurements are made of the same object the values obtained 

 will usually agree among themselves by less than 0.2 /x, and two 

 different experienced microscopists may be expected to obtain 

 values which will differ by less than this. Ewell 2 believes that 

 microscopic measurements may be relied upon as accurate among 

 themselves within less than o.i /x or even under exceptionally 

 favorable conditions within 0.05 /*. 



The degree of accuracy obtained will obviously be largely de- 

 pendent upon the resolving power of the objective employed. 



Micrometric measurements obtained with moderate magnifi- 

 cations are much more accurate as a rule than those obtained 

 with high powers. 



Method i. The method of direct comparison of object and scale 

 is generally impracticable and seldom available where ordinary 

 microscopes are employed, since it is next to impossible to have 

 object and scale lie in exactly the same plane under the micro- 

 scope. But in " micrometer" or " traversing" microscopes the 

 principle made use of is substantially that of a direct comparison 

 with a micrometer scale. 



Since the chemist-investigator is not infrequently called upon 

 to make long series of microscopic measurements of objects or 

 to measure the distance between lines in photographs of spectra, 

 etc., types of these special micrometric microscopes are shown in 

 Figs. 90, 91 and 92. 



For the comparison of lines in small spectra, scale rulings, 

 etc., the traversing microscope shown in Fig. 90 3 will be found 

 accurate and convenient. This instrument consists of two 

 microscopes A and B, mounted in fixed positions on a single 



1 Rogers, W. A., Proc. Am. Soc. Micros., 1883, 198. 



2 Ewell, J. Roy. Micr. Soc., 1910, 537. Nelson, ibid., 1910, 696. 



3 The comparator illustrated in Fig. 90 is manufactured by Carl Zeiss. For 

 methods for determining the corrections to be applied to micrometer microscopes, 

 consult Scientific Paper No. 215, U. S. Bureau Standards, by A. W. Gray, Microm- 

 eter Microscopes (1913). 



