MICROMETRY — MICROMETRIC MICROSCOPES 185 



in Fig. 117 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. 



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. 118 shows part 

 of the scale of a step micrometer. Instead of 

 being ruled in tenths and hundredths of a mil- 

 limeter as usual, such a value is used by Leitz 

 that when Leitz objectives are employed on a 

 Leitz microscope, it is only necessary to set 

 the draw-tube at the point indicated for that 

 particular objective. The ocular micrometer fig.iiS. Method Em- 

 value is obtained from a table, supplied with ployed in Ruling the 

 the instrument. Calibration by means of a Leitz Step Mlcrom - 



eter Ocular. 



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. 119, made by Zeiss, will be found useful. In 



place of line rulings, which would be 

 practically invisible, the scale con- 

 sists of a row of tiny black squares 

 touching at their corners. A scale 

 of this type will stand out sharply, 

 no matter how bright the object 

 may be. 



Filar Micrometers. — ■ In microm- 

 etry with oculars having fixed 



Fig. 119. Zeiss Contrast Microm- scaleg there ig a l ways the probability 

 eter Ocular for Measuring Bright . . . . , , 



Bodies> of considerable error, as we have 



