MICROMETRY — MICROMETRIC MICROSCOPES 



183 



mination of the ocular micrometer value may be made more 

 exact by eliminating fractions as shown in Fig. 116, 1 where it is 

 evident that a whole 

 number of ocular scale 

 divisions are included 

 in a whole number of 

 divisions of the stage 

 micrometer. This is 

 accomplished by alter- 

 ing the ratio between 

 the images of the two 

 scales through a change 

 in the position of the 

 draw-tube. Start with 

 the draw-tube extended 

 about half its total pos- 

 sible movement. Bring 

 the zero point of the 



Fig. 116. Micrometer Scales Properly Adjusted. 



ocular scale in contact with a line on the stage scale; focus 

 sharply. The relations of the images of the two scales will 

 now probably be essentially as indicated in Fig. 115. Note the 

 magnitude of the distance of the extreme line on the ocular 

 scale (50 on the B. & L. ocular) and the nearest line on the 

 stage micrometer. With this magnitude clearly in mind, push 

 in the draw-tube about 2 millimeters, focus and note whether 

 the ocular line (50) is now nearer or farther away than before. 

 If nearer push in the draw-tube a little more, focus and again 

 note the change. Keep this method up until both the zero 

 line and the farthest line on the ocular scale are each in con- 

 tact with lines on the stage scale. It is not essential that coin- 

 cidence must obtain in each ten divisions as indicated in Fig. 

 116. If on the other hand, the first change in draw-tube length 



1 Figs. 115 and 116 were drawn by means of a camera lucida and therefore show 

 exactly the conditions met with. Each division on the stage micrometer (the 

 lines crossing the entire field) equals o.i mm. With a pair of dividers compare 

 the magnitude of a space in Fig. 115 with one in Fig. 116. It will be found that 

 lengthening the draw-tube has changed the ratio between images of stage and 

 ocular scales. 



