I lS 



MAGNIFICATION AND MICROMETRY 



[CM. IV 



As object, place a scale of some kind ruled in millimeters on 

 the support under the magnifier. Put some white paper on the 

 table at the base of the support and on the side facing the light. 



Fig. 106. Ten Centimeter Rule. The upper edge is divided into milli- 

 meters, the lower into centimeters at the left and half centimeters at the right. 



Close one eye, and hold the head so that the other will be near 

 the upper surface of the lens. Focus if necessary to make the 

 image clear (§ 12). 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 others, the two images appearing to be fused in a single 

 visual field. 



§ 173. Measuring the Spread of Dividers. — Thisshould be 

 done on a steel scale divided to millimeters and i. 



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

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

 fier 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 3 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 mil- 

 limeters, then the magnification is 15-5-2=7^, that is the image 

 is f 1 /, times as long or wide as the object. In this case the image 

 is said to be magnified 7^ diameters, or 7^ times linear. 



