37 MAGNIFICA TION AND DRA WING. 



MAGNIFICATION AND DRAWING. 



§ 99. Measuring the Spread of Dividers. — This should be done 

 on a steel scale divided to millimeters and |ths. 



As \ mm. cannot be see plainly by the unaided eye, place one arm of 

 the dividers at a centimeter line, and then 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 (PI. I, Fig. 4, A' B'), 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 measuied 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 15-4-2=7^. That is, the image is 7^ times as 

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

 fied 7^ diameters, or 7^ times linear. 



The magnification of any simple magnifier may be determined ex- 

 perimentally in the way described for the tripod. 



MAGNIFICATION OF A COMPOUND MICROSCOPE. 



§ 100. The Magnification of a Compound Microscope is the 

 ratio between the final or virtual image (PI. I, Fig 5, B A), and the 

 object magnified (PL I, Fig. 5, A 3 B 3 ). 



The determination of the magnification of a compound microscope 

 may be made as with a simple microscope (§ 98), but this is very fa- 

 tiguing and unsatisfactory. 



§ 101. Stage, Object or Objective Micrometer. — For determining 

 the magnification of a compound microscope and for the purposes 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 on glass is most convenient. The spaces between 

 the lines should be -fa and y^j millimeter, and when high powers are 

 to be used the lines should be very fine. It is of advantage to have the 

 coarser lines filled with graphite (plumbago), especially when low pow- 

 ers are to be used. If one has an uncovered micrometer the lines may 

 be very readily filled by rubbing some of the plumbago on the surface 

 with the end of a cork, the superfluous plumbago may be removed by 

 using a clean dry cloth or a piece of the Japanese paper. After the 

 lines are filled and the plumbago wiped from the surface, the slide 



