CH. II'.] MAGNIFICATION AND MICROMETRY. 93 



the real image is 5 times as long as the object. The real images seen 

 in photography are mostly smaller than the objects, but the magnifica- 

 tion is designated in the same way by dividing the size of the real im- 

 age measured on the ground glass by the size of the object. For ex- 

 ample, if the object is 400 millimeters long and its image on the ground 

 glass is 25 mm. long, the ratio is 25 -=- 400 = j 1 ^- That is, the image 

 is yV n as long as the object and is not magnified but reduced. In 

 marking negatives, as with drawings, the sign of multiplication is put 

 bsfore the ratio, and in the example the designation would be x y^-th. 



MAGNIFICATION OF A SIMPLE MICROSCOPE. 



§ 147. The Magnification of a Simple Microscope is the ratio 

 between the object magnified (Fig. 16, A'B'), and the virtual image 

 (A'B 3 ). To obtain the size of this virtual image place the tripod mag- 

 nifier near the edge of a support of such a height that the distance from 

 the upper surface of the magnifier to the table is 250 millimeters. 



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

 port under the magnifier. Put some white paper on the table at the 

 base of the support, and on the side facing the light. 



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 

 (§ 9)- 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 binocular or double vision in which the 

 microscopic image is seen with one e} r e and the dividers with the other, 

 the two images appearing to be fused in a single visual field. 



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

 on a steel scale divided to millimeters and ^tbs. 



As \ mm. cannot be seen 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 (Fig. 16, A 3 B 3 ), and as the size of the ob- 

 ject is known, the magnification is determined by dividing the size of 

 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 



