CH. IV.] MAGNIFICATION AND MICROMETRY. 97 



dent, both may be reduced to the same denomination, thus : If the size 

 of the image is found to be gf mm. this number may be reduced to 

 tenths mm., so it will be of the same denomination as the object. In 9 

 mm. there are 90 tenths, and in £ there are 4 tenths, then the whole 

 length of the image is 90 + 4 = 94 tenths of a millimeter. The object 

 is 2 tenths of a millimeter, then there must have been a magnification 

 of 94 -r- 2 = 47 diameters in order to produce an image 94 tenths of a 

 millimeter long. 



Put the 25 mm. (1 in. C, or No. 4) ocular in place of one of 50 mm. 

 focus, and then put the camera lucida in position. Measure the size 

 of the image with dividers and a rule as before. The power will be 

 considerably greater than when the low ocular was used. This is be- 

 cause the virtual image (Fig. 21, B 8 A S ) seen with the high ocularis 

 larger than the one seen with the low one. The real image (Fig. 21, 

 A 1 !? 1 ) remains nearly the same, and would be just the same if positive, 

 par-focal oculars (§ 34, 68, note) were used. 



Lengthen the tube of the microscope 50-60 mm. by pulling out the 

 draw-tube. Remove the camera lucida, and focus, then replace the 

 camera and obtain the magnification. It will be greater than with the 

 shorter tube. This is because the real image (Fig. 94) is formed far- 

 ther from the objective when the tube is lengthened, and the objective 

 must be brought nearer the object (Fig. 94). The law is : The size of 

 object and image varies directly as their distance from the center of the 

 lens. The truth of this statement is illustrated by Figs. 93 and 94. 



§ 152. Varying the Magnification of a Compound Microscope. 

 It will be seen from the above experiments (§ 151) that independently 

 of the distance at which the microscopic image is measured (§ 153), 

 there are three ways of varying the power of a compound microscope. 

 These are named below in the order of desirability. 

 ( 1 ) By using a higher or lower objective. 



(2) By using a higher or lower ocular. 



(3) By lengthening or shortening the tube of the microscope (Fig. 94).* 

 § 153. Standard Distance of 250 Millimeters at which the Vir- 

 tual Image is Measured. — For obtaining the magnification of both 



* Amplifier. — In addition to the methods of varying the magnification given in 

 I 152, the magnification is sometimes increased by the use of an amplifier, that is 

 a diverging lens or combination placed between the objective and ocular and serv- 

 ing to give the image-forming rays from the objective an increased divergence. 

 An effective form of this accessory was made by Tolles, who made it as a small 

 achromatic concavo-convex lens to be screwed into the lower end of the draw-tube 

 (frontispiece) and thus but a short distance above the objective. The divergence 

 given to the rays increases the size of the real image about two-fold. 

 7 



