MICROMETRY MICROMETRIC MICROSCOPES 



in Fig. 95 in B. The value of the ocular micrometer scale must 

 be determined for each objective in turn, adjusting the draw- 

 tube in every case so as to avoid estimating fractions of a scale 

 division and in each case the record must be kept of the tube 

 length under which the observations were made. 



Correct 



Correct 



Incorrect 



FIG. 95. Determining the Ocular Micrometer Ratio: Heavy Lines = Stage 

 Micrometer, Light Lines = Ocular Micrometer. 



In the ordinary micrometer ocular it is often somewhat of an 

 eye and mental strain to count the number of scale divisions, 

 especially if the object is relatively large. To facilitate counting, 

 Leitz has placed upon the market a scale, part black, part light, 

 in which the divisions are sharply differentiated in blocks of 

 ten, both horizontally and vertically. This 

 type of ruling has received the name of Step 

 Micrometer, and is far less fatiguing to 

 employ than the older simple ruling. Fig. 

 96 shows part of the scale of a step microm- 

 eter. Instead of being ruled in tenths and 

 hundredths of a millimeter as usual, such a 

 value is used by Leitz that when Leitz ob- Fig- 96. Method Em- 

 jectives are employed on a Leitz microscope, P loye l in & the 



. . 111 Leitz Step Micrometer 



it is only necessary to set the draw-tube at Ocular, 

 the point indicated for that particular ob- 

 jective. The ocular micrometer value is obtained from a table, 

 supplied with the instrument. Calibration by means of a stage 

 micrometer is therefore unnecessary. 



For measuring bright or self-luminous bodies, such as the 

 incandescent filaments of lamps, etc., the Gebhardt Contrast 

 Micrometer, Fig. 97, made by Zeiss, will be found useful. In 

 place of line rulings, which would be practically invisible, the 

 scale consists of a row of tiny black squares touching at .their 



