102 MAGNIFICATION AND MICROMETRY. [CH. IV. 



§ 1 60. 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 § 159. 



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 (§ 148, 167). 



OCULAR MICROMETER. 



§ 161. Ocular Micrometer, Eye-Piece Micrometer. — This, as the 

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

 crometer 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 o^th 

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

 tate counting. If the micrometer is ruled in squares {net-micrometer) 

 it will be very convenient for many purposes. 



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 immediate^ 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 is measuring the size of the real 

 image, however, and the actual size of the object can only be deter- 

 mined 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. 



§ 162. Valuation of the Ocular Micrometer. — This is the value 

 of the divisions of the ocular micrometer for the purposes of microm- 

 etry, and is entirely relative, depending upon the magnification of the 

 real image formed by the objective, consequently it changes with every 

 change in the magnification of the real image, and must be specially 

 determined for every optical combination (z. e. , objective and ocular), 



