Ch. V] MEASURING WITH THE MICROSCOPE 151 



space instead of against one of the lines, consequently the size must 

 be estimated or guessed at rather thah really measured. 



§ 248. Micrometry by dividing the size of the image by the mag- 

 nification of the microscope. — For example, employ the 4 mm. ob- 

 jective, and 8x or lox ocular. For measurement use a preparation of 

 the blood corpuscles of the frog, necturus, or other animal with large 

 oval corpuscles. Obtain the size of the image of the long and short 

 axes of three corpuscles with the camera lucida and dividers, exactly 

 as in obtaining the magnification of the microscope (§ 234). Divide 

 the size of the image in each case 

 by the magnification, and the result 

 gives the actual size of the blood 

 corpuscles. Thus, suppose the im- 

 age of the long axis of the corpuscle 

 is 18 mm. and the magnification Fig. 95. Blood Preparation 



of the microscope 400 diameters ^"h * R™g around a Group 

 ,(.„,, 1 1 , , , , . OF Corpuscles. 



(§ 228), then the actual length of this 



long axis of the corpuscle is 18 mm. -f- 400 = 0.045 ^^- or 45/* (§ 231). 



Ap 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 (fig. 95). The differ- 

 ent corpuscles vary considerably in size, so that accurate comparison 

 of different methods of measurement can only be made when the same 

 corpuscles are measured in each of the ways. 



§ 249. 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. Meas- 

 ure the same three that were measured in § 248. 



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. 



