ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



853 



tively, should be of different formula), because of the very different 

 paths which the rays take in the two cases owing to the great differ- 

 ence in the lengths of the body-tubes. Both series are arranged to 

 give precisely the same magnifying powers, the difference in the body- 

 tubes being compensated for by the focal lengths. 



The settings are so adjusted in both series, that the lower focal 

 point of all the eye-pieces lies at the same plane when inserted 

 in the body-tube. No alteration of adjustment is, therefore, required 

 on changing the eye-piece, and the optical tube-length (i.e. the 

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

 lower one of the eye-piece) which is the standard factor for the 

 magnifying power, remains constant. This optical tube-length in 

 the Continental Microscopes (excluding small differences between the 

 various objectives) is equal to 180 mm., and in the English 270 mm., 

 provided that the length of the body-tube from the upper surface of 

 the setting of the objective, to the upper end of the tube on which 

 the eye-pieces rest, is 160 and 250 mm. respectively. 



The eye-piece 1 is only made for the Continental Microscopes, 

 and 27 only for the English, as the former would be too large for the 

 English body-tubes, while the latter would have an inconveniently 

 short focus with the Continental. 



The eye-pieces of unusually low power, designated " Finders," 

 serve the purpose of reducing to its lowest limits the available magni- 

 fication with each objective, thus facilitating the preliminary examin- 

 ation of specimens, and avoiding the labour of searching for particular 

 points with high powers. The Finder eye-piece 1 enables an 

 objective to be enijdoyed with its own proper magnifying power, i. e. 

 as if it were used as a magnifier without an eye-piece. In both the 

 diameter of the field of view amounts to fully a fifth of the focal 

 length of the objective used, with a relatively small angle — 12° in 1, 

 and 24° in 2. This is particularly favourable for rapid searches. 



These Finder eye-pieces are of special service with water- and oil- 

 immersion objectives, where great inconvenience is caused by having 

 to change an objective already adjusted for another of longer focus. 



The working eye-pieces for regular observation arc likewise of 



