Ch. V] 



MAGNIFICATION OF THE MICROSCOPE 



139 



§ 235. Varying the magnification of a micro- 

 scope. — There are five ways of varying the 

 power of a compound microscope : 



(i) 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. 



(4) By increasing or diminishing the dis- 

 tance at which the virtual image is projected 

 (fig- 8s): 



(5) By changing the relative position of the 

 combinations in an adjustable objective (§ 31, 

 134) or by the use of an amplifier (§ 235a). 



§ 235a. Amplifier. — In addition to the methods of 

 varying the magnification given in § 235, the magnifica- 

 tion is sometimes increased by the use of an aniphfier, 

 that is, a diverging lens or combination placed between 

 the objective and ocular and serving to give the image- 

 forming rays from the objective an increased diverg- 

 ence. An effective form of this accessory was made 

 by ToUes, who made it as a small achromatic con- 

 cavo-convex lens to be screwed into the lower end of 

 the draw-tube (fig. 25) and thus but a short distance 

 above the objective. The divergence given to the 

 rays usually increases the size of the real image about 

 twofold. 



§ 236. Standard distance at which the vir- 

 tual image is measured. — For obtaining the 

 magnification of both the simple and the com- 

 pound microscope the directions were to meas- 

 ure the virtual image at a distance of 250 

 millimeters. This is because some standard 

 distance must be chosen so that different 

 workers can compare their results. The mag- 

 nification could be found at almost any dis- 

 tance, and in getting the magnification of 

 drawings the image distance is rarely exactly 

 250 milHmeters. Whenever the magnification 

 of the microscope as a whole or of the objec- 

 tive or the ocular is mentioned, however, it is 



Image 



Object 



Fig. 84. To Show 



THAT THE SiZE OF THE 



Real Image Depends 

 UPON ITS Relative 

 Distance from: the 

 Center of the Ob- 

 jective. 



Object I The object 

 one unit of distance 

 from the center of the 

 lens (CL). 



Image i, 2,3, 4 The 

 image four units of dis- 

 tance from the lens and 

 hence four times as long 

 as the object. 



