104 



MAGNIFICATION AND MICROMETRY 



[CH. IV 



eye or measured as if it were an actual object. For example, suppose 

 the object were three millimeters long and its image on the ground glass 

 measured 15 mm., then the magnification must be, 15-5-3 = 5, that is, 

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

 in photograph}- 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 

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

 ground glass is 25 mm. long, the ratio is 25-f-400=- 1 1 ^-. That is, the 

 image is j 1 ,. th as long as the object and is not magnified but reduced. 

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

 put before the ratio, and in the example the designation would be 



rth. 



MAGNIFICATION OF A SIMPLE MICROSCOPE 



§ 156. The Magnification of 

 a Simple Microscope is the ratio 

 between the object magnified (Fig. 

 16, A'B'), and the virtual image 

 (A 3 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 mag- 

 nifier to the table is 250 millimeters. 



Fig. 94. Tripod Magnifier. 



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. 95. Ten Centimetei Rule. The upper edge is divided into millimeters, 

 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 



