152 MEASURING WITH THE MICROSCOPE [Ch. V 



This simply enables one to put the image of a fine rule on the image 

 of a microscopic object. It is theoretically an excellent method, and 

 nearly the same as measuring the spread of the dividers with a simple 

 microscope (§ 231). 



§ 250. Micrometry with the ocular micrometer with fixed lines. — ■ 

 Use the 4 mm. objective, and the ocular with the ocular micrometer. 

 For object use the same corpuscles as in § 248-249. Make sure that 

 all the conditions are exactly as when the valuation was determined; 

 then put the preparation under the microscope and find the same three 

 red corpuscles that were measured in the other ways (§ 248). 



Count the divisions on the ocular micrometer required to enclose 

 or measure the long and the short axis of each of the corpuscles, 

 multiply the number of spaces in both cases by the valuation of the 

 ocular micrometer, and the results will represent the actual length of 

 the axes of the corpuscles in each case. 



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 obtained by each method. 



§ 251. Micrometry with the movable scale ocular micrometer. — 

 Use the same preparation and objective as before. Arrange the 

 micrometer ocular so that the long axis of the corpuscle will coincide 

 with the cross line in the micrometer scale (fig. 91-92). Get one end 

 of the corpuscle exactly level with one division of the micrometer 

 scale. Note the position of the drum, and then rotate it until the 

 other end of the corpuscle is exactly against the nearest line of the 

 micrometer. Count up the entire intervals required and the partial 

 interval on the drum. Suppose it requires 5 entire and 0.60 inter- 

 vals (see explanation of fig. 92); then the whole corpuscle must be 

 5.60 intervals multiplied by 4ft (§ 242) the value of one interval; 



5.6X4= 22-4JU- . 



§ 252. Micrometry with the filar micrometer. — Use the same 

 preparation and objective as before, but use a filar micrometer. Note 

 how many graduations on the recording comb and drum (fig. 93) are 

 required to measure each dimension of the corpuscle, and multiply 

 by the valuation as in the other cases. 



