92 



division lines downwards. The values of divisions are best determined by comparison 

 with a good stage-micrometer for each combination of objective and eye-piece. 

 Approximate values, which are in many cases sufficiently accurate, will be 

 found in tables supplied with each eye-piece micrometer. These tables apply to 

 combinations of achromatic objectives with HUYGENIAN eye-pieces, and are 

 calculated for the prescribed (160 mm) tube-length. 



In combinations of the compensating eye-pieces 6 with any of the apo- 

 chromatic objectives the approximate determination of the value of an interval 

 becomes very much simplified. The divisions in this eye-piece are so computed 

 that with a tube-length of 160 mm the value of one interval of the micrometer 

 scale represents exactly as many micra, /u (0.001 mm = 0.00003937 in.), as there 

 are millimetres in the focal length of the objective, i. e. , Apochromatic lens: 

 16 mm, 8 mm &c., value of interval: 16 (JL, 8 /u &c. (See "Zeitschr. f. wissen. Mi- 

 krosk.", vol. V, 1888, pp. 150-155.) 



As however these values also are still only approximate, a comparison 

 must be made with the stage-micrometer whenever more exact measurements 

 are required. 



Our regular manufacture of Eye-piece Micrometers is restricted to the 

 one form, catalogued as No. 117: 5 mm divided in Vio 1*1111. 



Price, incl. case: Marks 5. . Code-word: Favilon. 

 Other divisions on application. 



When ordering eye-piece micrometers please state whether they are required 

 for ordinary or for micrometer eye-pieces, 



Two forms of Stage Micrometers are regularly supplied: 



No. 118: 1 mm divided in 100 parts. 



Price, incl. case: Marks w. . Code-word: Pavimiento. 



No. 119: 1 cm divided in millimetres, the last in tenths. 



Price, me 1. case: Marks 6. . Code- word : Pa von. 



With regard to eye-piece and stage cross-line micro- 

 meters refer to pp. 96 98. 



Eye-pieces with sliding eye-lenses, so-called "micro- 

 meter eye-pieces", are constructed for the purpose of 

 facilitating exact adjustment of the micrometer scale to 

 the eyes of individual observers. These are the HUYGENIAN 

 eye-pieces 2 and 3 for achromatic and compensating 

 eye-piece 6 for apochromatic objectives. Fig. 51. 



Fig. 51. Micrometer 

 Eye-piece. (Full Size.) 



