11 



The mounts of the eye-pieces are so adjusted that the lower focal points 

 of all the numbers He in the same plane when the eye-pieces are inserted into 

 the body- tube (Fig. 3). 



Ocular No. 2 



12 



18 



Fig. 3. 

 Compensating Eye-pieces. (V 2 Full Size.) 



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



B B. Lower focal plane of the eye-pieces. 



We now supply the eye-pieces 8 and 72 in mounts differing somewhat from those shown in the 

 illustration in order that they may also be used with our drawing apparatiis. 



A change of eye -pieces necessitates therefore scarcely any alteration of 

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

 lower focal point of the eye-piece remains constant. This distance, which is the 

 determining element of the sum of the magnifying power, is designated "the 

 optical tube-length" (see above-quoted work by DIPPEL, p. 188). 



In continental microscopes with apochromatic objectives and compensating 

 eye-pieces this optical tube-length is 180 mm (about T 1 /^ in.), irrespective of 

 small differences between various objectives, the tube-length proper (see page 3) 

 being in all cases maintained at 160 mm (about C 1 /^ in.). 



The classification of these eye-pieces is carried out on the plan proposed 

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

 an eye-piece, when used with the standard tube-length, increases the initial 

 magnifying power of an objective afford a correct measure of the eye-piece 

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

 of distinguishing numbers. In this way the series of our compensating eye- 

 pieces has been arranged by the eye-piece 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 eye-piece with any apochromatic objective may thus be quickly 



