CH. IV] MAGNIFICATION AND MICROMETRY 129 



be made when the same corpuscles are measured in each of the 

 ways. 



§ 185. 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- 

 corpuscles. Measure the same three that were measured in § 184. 



Remove "the object, place the stage micrometer under the 

 microscope, 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 (§ 173, 197). 



OCULAR MICROMETER 



§ 186. 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 equi- 

 distant and T V or ^ mm. apart, every fifth line should be longer 

 and heavier to facilitate counting. If the micrometer is ruled in 

 squares {net micrometer) it will be very convenient for many pur- 

 poses. 



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

 the form of the ocular (i. e., 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 microme- 

 ter, 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. 



