104 



MAGNIFICATION AND MICROMETRY. 



[CH. IV. 



raise or lower the eye-lens to make them so ; that is, focus as with the 

 simple magnifier. 



When the lines of the ocular micrometer are distinct, focus the mi- 

 croscope (§ 45, 56) for the stage micrometer. The image of the stage 

 micrometer will appear to ba directly under. or upon the ocular microm- 

 eter. 



Fig. 100. Ocular Screw Micrometer with 

 compensation ocular 6. The upper figure 

 shows a sectional view of the ocular and the 

 screw for moving the micrometer at the right. 

 At the left is shown a clamping screw to 

 fasten the ocular to the upper part of the mi- 

 croscope tube. Below is a face view, showing 

 the graduation on the wheel. An ocular mi- 

 crometer like this is in general like the cob- 

 iveb micrometer and may be used for measur- 

 ing objects of varying sizes very accurately. 

 With the ordinary ocular micrometer very 

 small objects frequently fill but a part of an 

 interval of the micrometer, but with this the 

 movable cross lines traverse the object [or rath- 

 er its real image ) regard/ess of the minuteness 

 of the object. (Zeiss' Catalog, No. 30). 



§ 164. Make the lines of the two micrometers parallel by rotating the 

 ocular, or changing the position of the stage micrometer, or both if nec- 

 essary, and then make any two lines of the stage micrometer coincide 

 with any two on the ocular micrometer. To do this it may be necessary 

 to pull out the draw-tube a greater or less distance. See how many 

 spaces are included on each of the micrometers. 



Divide the value of the included space or spaces on the stage microm- 

 eter by the number of divisions 011 the ocular micrometer required to 

 include them, and the quotient so obtained will give the valuation of 

 the ocular micrometer in fractions of the unit of measure of the stage 

 micrometer. For example, suppose the millimeter is taken as the unit 

 for the stage micrometer and this unit is divided into spaces of T \yth and 

 TTo-th millimeter. If now, with a given optical combination and tube- 

 length, it requires 10 spaces on the ocular micrometer to include the 

 real image of yij-th millimeter on the stage micrometer, obviously one 

 space on the ocular micrometer would include only one-tenth as much, 

 or y'j-th mm. -*- 10 = T J mm. That is, each space on the ocular mi- 

 crometer would include T ^ 7) - of a millimeter on the stage micrometer, 

 or T ^ T th millimeter of length of any object under the microscope, the 

 conditions remaining the same. Or, in other words, it would require 



