===== 17 



The mounts of the oculars are adjusted so that the lower focal points of 

 all the numbers lie in the same plane when they are inserted into the body-tube 

 (Fig. 4). 



Ocular N. 2 



12 



18 



Fig. 4. 

 Compensating 1 Oculars. ('/, Full Size.) 



A A. Plane of the upper edge of the tube. 



B B. Lower focal plane of the oculars. 



A change of oculars necessitates therefore scarcely any alteration of focus, 

 the distance between the upper focal point of the objective and the lower focal 

 point of the ocular remaining constant. This distance, which is the determining 

 element of the total magnifying power, is designated "the optical tube- 

 length", and is designated by A in Fig. 1, p. 9, (see above-quoted work by 

 DIPPEL, p. 188). 



In continental microscopes with apochromatic objectives and compensating 

 oculars this optical tube-length is 180 mm (about 7 Vie in.), irrespective of 

 small differences between various objectives, the tube-length proper (see p. 8) 

 being in all cases maintained at 160 mm (about 6Y 2 in.). 



The classification of these oculars is carried out on the plan proposed 

 by ABBE. Accordingly, the numbers, which denote how many times an ocular 

 - when used with the standard tube-length - - increases the initial magnifying 

 power of an objective, afford a correct measure of the ocular magnifications 

 and, at the same time, furnish the figures for a rational series of distinguishing 

 numbers. In this way the series of our compensating oculars has been arranged 

 "by the ocular magnifications 2. 4, 6, 8, 12, 18, which supply, at the same time, 

 the distinguishing numbers. 



The magnification at an image distance of 250 mm (10 in.) obtained by the com- 

 bination of a compensating ocular with any apochromatic objective may thus be quickly 

 found by multiplying its number by the numerical value of the initial magnification of 

 the objective. For instance, an objective of 3 mm focus combined with ocular 12 gives 

 a magnification of 12 X 83 -3 = 1000 diameters. 



Swiss, Sena. 



