ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



325 



Mr. Nelson considers 25 and 20 threads upon the screw will pro- 

 vide a convenient fine focusing movement for students' Microscopes, 

 though, of course, any desired speed can be obtained by proper com- 

 bination of the threads. For instance, 32 and 30 would give 1/480 in. 

 for each revolution, and 31 and 30 would give 1/960 in. He thinks 

 the system specially commendable, from the fact that fine movements 

 are thus obtained by the use of strong screws having coarse threads. 

 In his opinion the difficulty with the usual fine adjustments applied 

 to Continental Microscopes has been " that if there is a direct-acting 

 screw with its thread fine enough to give a sufficiently slow move- 

 ment, then the screw will be found too weak to stand the wear and 

 tear. On the other hand, if it is strong enough to stand the wear 

 and tear, the screw will have to be too quick," 



It should be noted that Herr E, Gundlach applied a differential 

 fine adjustment of this kind to the Microscope some years ago, and 

 that at the Inventions Exhibition of last year Messrs. Eoss and Co. ex- 

 hibited a differential movement specially devised by Dr. H. Schroder 

 for apj)lication to Microscopes having the " Jackson " limb. 



Anderson's Double-action Fine Adjustment.— Messrs. Anderson 

 & Sons have devised a fine adjustment by which two different rates 

 of speed in focusing are provided, the one acting on the lever at the 

 rate of 40 turns to the inch, and the other at 100 to the inch. 



The mechanism is shown in fig. 62, 

 where A is a stud carrying the usual 

 tube nose-piece as applied to the 

 " Jackson " form of fine adjustment, 

 with a swinging pin D passing loosely 

 through, and suspended on the top 

 of, the metal block C, which slides 

 freely in parallel fittings and termi- 

 nates below in a knife-edge. B is a 

 long lever acting on C. S is a screw 

 having 40 threads to the inch at the 

 lower part and 100 above ; it is fixed 

 in a hinged shoe-piece G, which covers 

 a rigid bar projecting from the side of 

 the body-tube support. 



In action the rotation of the milled 

 head E upwards raises the lever B, and consequently C, D, and A 

 (the latter carrying the tube nose-piece), at the rate of 40 turns to 

 the inch, and when a slower motion is required, the rotation down- 

 wards of the milled head F draws up the screw and shoe-piece G 

 together with E, B, C, D, and A, at the rate of 100 turns to the inch, 

 the rigid piece within G serving as the stop for this motion. A spiral 

 spring within the body-tube acts against the upward movement of the 

 lever B, and therefore opposes the screw movements of the milled 

 heads E and F. 



Fritsch's Stage for Stereoscopic Photo-micrographs.— Dr. G. 

 Fritsch's apparatus, to which we referred at p. 144, is an elaboration 



