42 



MAGNIFICA TION AND DRA WING. 



\ 109. Micrometry by the use of a stage micrometer on which to mount the ob- 

 ject. — In this method the object is mounted on a micrometer and then put under 

 the microscope and the number of spaces covered by the object is read off directly. 

 It is exactly like putting any large object on a rule and seeing how many spaces of 

 the rule it covers. The defect in the method is that it is impossible to properly 

 arrange objects on the micrometer. Unless the objects are circular in outline they 

 are liable to be oblique in position and in every case the end or edges of the object 

 may be in the middle of a space instead of against one of the lines, consequently 

 the size must be estimated or guessed at rather than really measured. 



§ 1 10. Micrometry by dividing the size of the image by the magnifi- 

 cation of the microscope. — For example, employ the 3 mm. objective, 25 

 mm. .ocular, and a Necturus' red blood-corpuscle preparation as object.* 

 Obtain the size of the image of the long and short axes of three cor- 

 puscles with the camera lucida and dividers exactly as in obtaining the 

 magnification of the microscope (§ 102). Divide the size of the image 

 in each case by the magnification and the result will be the actual size 

 of the blood-corpuscle. Thus, suppose the image of the long axis of 

 the corpuscle is 18 mm. and the magnification of the microscope 400 

 diameters (§ 97), then the actual length of this long axis of the cor- 

 puscle is 18 mm. -^400 = .045 mm. or 45 /a (§ 108). 



§ in. 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 § no. 



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 meas- 

 ured. 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 mi- 

 crometer 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 (§§ 99, 117). 



OCULAR MICROMETER. 



§ 112. Ocular Micrometer, Eye- Piece Micrometer. — This, as 

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



* As the same three blood corpuscles are to be measured in three ways, it is an 

 advantage to put a delicate ring around a group of three or more corpuscles and 

 make a sketch of the whole enclosed group, marking on the sketch the corpuscles 

 measured. 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. 



