ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



525 



Fig. 116. 



from the stage. In some cases, however, the mirror is not of the 

 right focus and the latter course cannot be adopted. The appearance 

 should be like B as nearly as possible. 



Many objectives have front lenses much larger than is necessary. 

 This is a real detriment, for light is admitted by the outer zone, which 

 has nothing to do with forming the image ; and although it does not 

 reach the eye directly, yet it is reflected again and again from the 

 various lens-surfaces of the objective, forming a haze. Gundlach, in 

 some of his recent objectives, evidently recognizing this fact, turns 

 down the lens-front to the mere size actually used. Where the 

 working distance is sufficient, a diaphragm cap would effect the same 

 purpose. Upon this principle also is the action of a substage dia- 

 phragm of suitable size (about the size of the field). Especially is it 

 frequently the case with lenses of large aperture and short working 

 distance that only a small proportion of the front surface is used. 



Bertrand's Adapter Nose-piece. — This adapter was devised by 

 M. E. Bertrand, now Secretary of the Academic des Sciences, Paris, to 

 facilitate the rapid change of objectives. It consists of a short tube 

 having an internal thread to screw on the ordi- 

 nary French nose-piece ; the tube extends 

 below in a broad thick flange, on either side of 

 which is a U-shaped spring. The under sur- 

 face of the flange is cut out slightly in front 

 to permit the entry of a shallow flange-ring, 

 with which each objective is furnished in place 

 of the usual screw. The flange-ring on the 

 objective slides between the adapter and the 

 springs, and to insure correct centering, a 

 short collar on the upper face of the flange is pressed by the springs 

 into a corresponding cylindrical hollow in the adapter. 



This system of adapter does not appear to have been devised for 

 general use, but only for the series of objectives used with M. Ber- 

 trand's Petrological Microscope.* 



Rings for throwing the Coarse Adjustment out of gear. — 

 Messrs. Beck now supply the rings shown in 

 fig. 117 for preventing the inadvertent use of 

 the coarse adjustment at soirees, &c,, when an 

 object is being shown with a high power. It 

 consists of a cut ring with a screw-thread 

 which is passed over the axis of the milled 

 heads of the coarse adjustment, and a second 

 deeper ring, also with a screw thread, which 

 fits over the milled heads, and to which the 

 cut ring can be screwed. The deep ring has 

 on its outer side a flange which forms with 

 the inner circumference of the cut ring a deep 

 groove rather wider than the milled head. This prevents the ring 

 from being moved laterally, whilst at the same time it fits loosely 



* See this Journal, iii. (1883) p. 413. 



Fig. 117. 



