1 1 4 MA GNIFICA TION AND MIC ROME TR V [ CH. IV 



marking on the sketch the corpuscles measured (Figs. 61-66). The 

 different corpuscles vary considerably in size, so that accurate com- 

 parison of different methods of measurement can only be made when 

 the same corpuscles are measured in each of the ways. 



§ 169. Micrometry by the use of a Stage Micrometer and a 

 Camera Lucida. — Employ the same object, objective and ocular as 

 before. Put the camera lucida in position, and with a lead pencil 

 make dots on the paper at the limits of the image of the blood- 

 corpuscle. Measure the same three that were measured in § 16S. 



Remove the object, place the stage micrometer under the micro- 

 scope, focus well, and draw the lines of the stage micrometer so as to 

 include the dots representing the limits of the part of the image to be 

 measured. As the value of the spaces on the stage micrometer is 

 known, the size of the object is determined by the number of spaces 

 of the micrometer required to include it. 



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 (§ 157, 176). 



OCULAR MICROMETER 



§ 170. Ocular Micrometer, Eye-Piece Micrometer. — This, 

 as the name implies, is a micrometer to be used with the ocular. It is 

 a micrometer on glass, and the lines are sufficiently coarse to be clearly 

 seen by the ocular. The lines should be equidistant and about T Vth or 

 7,-Vth mm. apart, and every fifth line should be longer and heavier to 

 facilitate counting. If the micrometer is ruled in squares {net micro- 

 meter) it will be very convenient for many purposes. 



The ocular micrometer is placed in the ocular, no matter what the 

 form of the ocular (z. <?., whether positive or negative) at the level at 

 which the real image is formed by the objective, and the image appears 

 to be immediately upon or under the ocular micrometer, and hence 

 the number of spaces on the ocular micrometer required to measure 

 the real image may be read off directly. This, however, is measuring 

 the size of the real image, and the actual size of the object can only be 

 determined by determining the ratio between the size of the real image 

 and the object. In other words, it is necessary to get the valuation of 

 the ocular micrometer in terms of a stage micrometer. 



§ 171. Valuation of the Ocular Micrometer. — This is the 

 value of the divisions of the ocular micrometer for the purposes of 



