39 MAGNIFICA TION AND DRA WING. 



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. 5, B' A'), seen with the high ocular is 

 larger than the one seen with the low one. The real image (Fig. 5, A 

 B), remains nearly the same, and would be just the same if positive, 

 par-focal oculars (§§ 21, 48 note), were used. 



Lengthen the body 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 body. This is because the real image (Fig. 5, B A) is formed 

 farther from the objective when the body is lengthened, and being 

 formed farther from the objective it must necessarily be larger (§ 7 and 

 Fig. 28). 



§ 103. Varying the Magnification of a Compound Microscope. 

 It will be seen from the above experiments (§ 102), that independently 

 of the distance at which the microscopic image is measured (§ 104), 

 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* 



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

 tual 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 be- 

 cause some standard must be selected, and this is the most common 

 one. The necessity for the adoption of some common standard will be 

 seen at a glance in PL III, Fig. 28, where is represented graphically 

 the fact that the size of the virtual image depends directly on the dis- 

 tance at which it is projected, and this size is directly proportional to 

 the vertical distance from the apex of the triangle, of which it forms a 

 base. The distance of 250 millimeters has been chosen on the suppo- 

 sition that it is the distance of most distinct vision for the normal 

 human eye. 



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

 \ 103, 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. 

 This accessory was first made effective by Tolles, who made it in the form of a 

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

 draw-tube (PI. II, Fig. 10) and thus but a short distance above the objective. The 

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



