CH. IV] MAGNIFICATION AND MICROMETRY 117 



( 60), it will be remembered that it forms a real image on the 

 ground glass placed on the top of the tube, and that this real image 

 could be looked at with the eye or measurered 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 is i5-=-3=5, that is, 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 magnification is designated 

 in the same way by dividing the size of the real image measured on 

 the ground glass by the size of the object. For example, if the ob- 

 ject is 400 millimeters long and its image on the ground glass is 25 

 mm. long the ratio is 25-7-400 y 1 ^. That is, the image is y 1 ^ 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 is X T ^. In photog- 

 raphy (Ch. VIII) and when using the magic lantern and the pro- 

 jection microscope the images are real, and may be measured on the 

 screen as if real pictures. 



MAGNIFICATION OF A SIMPLE MICROSCOPE 



172. The Magnification of a Simple Microscope is the 

 ratio between the object magnified (Fig. 16, A 1 !?'), and the virtual 



FIG. 105. Tripod Magnifier 



image (A 3 B 3 ). To obtain the size of this virtual image place the 

 tripod magnifier 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. 



