CH. IV~\ MAGNIFICATION AND MICROMETRY 123 



high ocular is larger than the one seen with the low one. The real 

 image (Fig. 26 A 1 B 1 ) remains nearly the same, and would be just 

 the same if positive, par-focal oculars (§ 43, 82, 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 is greater than with 

 the shorter tube. This is because the real image (Fig. no) is 

 formed farther from the objective when the tube is lengthened, and 

 the objective must be brought nearer the object. 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. 109 

 and no. 



§ 177. Varying the Magnification of a Compound Micro- 

 scope. — It is seen from the above experiments (§ 176) that in- 

 dependently of the distance at which the microscopic image is 

 measured (§ 178), there are three ways of varying the power of a 

 compound microscope. These are named below in the order of 

 desirability. 



(1) By rising a higher or lower objective. 



( 2 ) By using a higher or lower ocular. 



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

 no).* 



§ 178. Standard Distance of 250 Millimeters at which the 

 Virtual Image is Measured. — For obtaining the magnification of 

 both the simple and the compound microscope the directions were 

 to measure the virtual image at a distance of 250 millimeters. This 

 is not that the image could not be seen and measured at any other 

 distance, but because some standard must be selected, and this is 

 the most common one. The necessity for the adoption of some com- 

 mon standard will be seen at a glance in Fig. 111, where is repre- 



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

 given in \ 177, the magnification is sometimes increased by the use of an 

 amplifier, that is a diverging lens or combination placed between the objec- 

 tive and ocular and serving 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. 



