CH. IV] 



MAGNIFICATION AND MICROMETRY 



117 



The same corpuscle is, of course, of the same actual size, when 

 measured in each of the three ways, so that if the methods are correct 

 and the work carefully enough done, the same results should be ob- 

 tained by each method. (§ 176).* 



Fig. 106. Ocular Screw-Micrometer with 

 compensation ocular X 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 

 micrometer like this is in general like the 

 cob-web -micrometer and may be used J'or 

 measuring objects of varying sizes very accu- 

 rately. 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 

 rather its real image) regardless of the minute- 

 ness of the object. (Zeiss' Catalog). 



'',, 174. Obtaining the Valuation of the Screw or Filar Micrometer. — This 

 micrometer (Fig. 106-107 ) usually consists of a Ramsden's ocular and cross lines. 

 As seen in Fig. 107^ there are three lines. The horizontal and one vertical line 

 are fixed. One vertical line may be moved by the screw back and forth across the 

 field. 



For obtaining the valuation of this ocular micrometer an accurate stage mi- 

 crometer must be used. Carefully focus the yjifth mm. spaces. The lines of the 

 ocular micrometer should also be sharp. If the3" are not focus them by moving 

 the top of the ocular up or down ( \ 172 ). Make the vertical lines of the filar mi- 

 crometer parallel with the lines of the stage micrometer. Take the precautions 

 regarding the width of the stage micrometer lines given in | 176 (see also Fig. 

 10S). Note the position of the graduated wheel and of the teeth of the recording 

 comb, and then rotate the wheel until the movable line traverses one space on the 

 stage micrometer. Each tooth of the recording comb indicates a total revolution 

 of the wheel, and by noting the number of teeth required and the graduations on 

 the wheel, the revolutions and parts of revolution required to measnre the Ti nyth 

 mm, of the stage micrometer can be easily noted. Measure in like manner 4 or 5 

 spaces and get the average. Suppose this average is i^th revolutions or 125 

 graduations on the wheel, to measure the rnffth mm. or io/< (see \ 166), then one 

 of the graduations on the wheel would measure ion divided by 125 =.oS/<. In 

 using this valuation for actual measurement, the tube of the microscope and the 

 objective must be exactly as when obtaining the valuation (see 2 175 ). 



Example of Measurement. — Suppose one uses the red blood corpuscles of a dog 

 or monke3', etc., every condition being as when the valuation was determined, one 

 notes very accurately how many of the graduations on the wheel are required to 

 make the movable line traverse the object from edge to edge. Suppose it requires 



