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 
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. 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- gcales there is always the probability 
eter Ocular for Measuring Bright . • 1 i 1 1 
01 considerable error, as we have 
Fig. 118. Method Em¬ 
ployed in Ruling the 
Leitz Step Microm¬ 
eter Ocular. 
