CH. IV] MA GNIFICA TION AND MICRO ME TRY 105 



(§ 11). Open the closed eye and the image of the rule will appear as 

 if on the paper at the base of the support. Hold the head very still, 

 and with dividers get the distance between any two lines of the image. 

 This is the so-called method of double vision in which the microscope 

 image is seen with one eye and the dividers with the other, the two 

 images appearing to be fused in a single visual field. 



§ 157. Measuring the Spread of Dividers. — This should be 

 done on a steel scale divided to millimeters and iths. 



As \ mm. cannot be seen plainly by the unaided eye, place one 

 arm of the dividers at a centimeter line, and with the tripod magnifier 

 count the number of spaces on the rule included between the points of 

 the dividers. The magnifier simply makes it easy to count the spaces 

 on the rule included between the points of the dividers — it does not, of 

 course, increase the number of spaces or change their value. 



As the distance between any two lines of the image of the scale 

 gives the size of the virtual image (Fig. 16, A :5 B 3 ), and as the size of 

 the object is known, the magnification is determined by dividing the 

 size of the image by the size of the object. Thus, suppose the distance 

 between the two lines of the image is measured by the dividers and 

 found on the steel scale to be 15 millimeters, and the actual size of the 

 space between the two lines of the object is 2 millimeters, then the 

 magnification must be 15^2=7^. That is, the image is 7^2 times as 

 long or wide as the object. In this case the image is said to be 

 magnified 7^ diameters, or 7^ times linear. 



The magnification of any simple magnifier may be determined 

 experimentally in the way described for the tripod. 



MAGNIFICATION OF A COMPOUND MICROSCOPE 



§ 158. The Magnification of a Compound Microscope is the 

 ratio between the final or virtual image (Fig. 21, B 3 A 3 ), and the object 

 magnified (A B). 



The determination of the magnification of a compound microscope 

 may be made as with a simple microscope (§ 156), but this is very 

 fatiguing and unsatisfactory. 



£ 159. Stage, Object or Objective Micrometer. — For deter- 

 mining the magnification of a compound microscope and for the 

 purpose of micrometry, it is necessary to have a finely divided scale 

 or rule on glass or on metal. Such a finely divided scale is called a 

 micrometer, and for ordinary work one mounted on a glass slide 

 (1X3 in, 25 X 76 mm.) is most convenient. 



