THE MICROSCOPE. 47 



of objectives and oculars that may be placed on the micro- 

 scope. Such a micrometer consists of a glass slide with a 

 very fine scale engraved on it with the aid of a machine. 

 In the English micrometer the divisions are l0 1 OQ ths and 

 T -g-<j-ths of an inch. In the French micrometer they are 

 'i^ths of a millimeter. . Measurements are easiest with 

 the latter. 



Let the -j-in. objective and the ordinary ocular be on the 

 microscope (the No. 7 objective and No. 3 ocular in the 

 case of Hartnack's microscope), with the tube fully elon- 

 gated, what is the magnifying power? Place a millimeter 

 micrometer on the stage, and focalise five of its larger 

 divisions, the space included is 0-05 millimeter. The 

 magnified image requires in some way or other to be com- 

 pared with an ordinary hand millimeter scale to determine 

 how much the 0-05 MM has been magnified. Every one 

 should try to do this as follows : Hold the points of a pair 

 of compasses ten inches from the eye (on a level with the 

 stage of Hartnack's microscope), keep both eyes open, look 

 at the micrometer lines with one, and at the points of the 

 compasses with the other eye, open or close the points 

 until they include exactly the five divisions of the micro- 

 meter, then apply the compasses to a millimeter scale. 

 Suppose they cover exactly 1 5 millimeters, the magnifying 

 power is ascertained by dividing the apparent size (15 MM) 

 by the real size (0*0 5 MM). 



Apparent Size. Real Size. Magnifying Power. 



I5MM -r- o'osMM = 300 diameters. 



If an English micrometer be used, bring into focus a 

 -roVrrth mcn division. Suppose it magnified to 0-3 inch, 

 then; 



0*3 inch -7- o'ooi inch = 300 diameters. 

 The use of compasses in this process is not essential ; 

 the millimeter, or inch scale itself, may be held at the side 

 of the microscope on a level with the stage. The process, 

 however, is much easier with compasses, because their 

 points are more readily seen, and, unlike the lines of the 

 scale, they are not apt to produce confusion. 



