Ch. V] MAGNIFICATION BY THE MICROSCOPE 133 



or drawing has the width or length of every detail 450 times as great 

 as the object. 



§ 229. Magnification of real images. — In this case the magnifi- 

 cation is the ratio between the size of the real image and the size of 

 the object, and the size of the real image can be measured directly. 

 By recalUng the work on the function of an objective, it will be remem- 

 bered 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 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 is 15 -^ 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 magnifi- 

 cation 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 object is 400 millimeters long and its image on the 

 ground-glass is 25 millimeters long, the ratio is 25 -^ 400 = yV- 

 That is, the image is xV 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 designa- 

 tion is XtV- In photography (Ch. VII) and when using the magic 

 lantern and the projection microscope the images are real, and may 

 be measured on the screen as if real pictures (fig. 79). 



§ 230. The magnification of a simple microscope is the ratio be- 

 tween the virtual image (fig. 6, 77, A^B^) and the object magnified 

 (A'B^). To obtain the size of this virtual image, place the tripod 

 magnifier near the edge of a support or block 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 

 support 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 wiU be near the 

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

 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 



