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 : 



(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. 



(4) By increasing or diminishing the dis- 

 tance at which the virtual image is projected 

 (fig. 85). 



(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 amplifier, 

 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 Tolles, 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 millimeters. 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 SlZE 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. 



