ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



685 



end is allowed to hang down in the water, where it becomes thoroughly 

 saturated and untwisted. It is then laid across the fork, and dropped 

 into its furrows in the manner above described, the little lead weights 

 exerting a definite tension. 



Varnish * is immediately applied to secure the webs, and the 

 frame is not touched till it is dry. 



The bevel-edge of the web-frame introduced by Repsold offers 

 great facilities for accurate webbing, and should, Mr. Gill says, be 

 employed in all future micrometers. 



Schroder's Differential-screw Fine Adjustment. — This device by 

 Dr. H. Schroder was exhibited by Messrs. Eoss in the Inventions 

 Exhibition of 1885, and is shown in figs. 140 and 141. 



Fig. 140. 



Fig. 141. 



The nose-piece A is attached to a tube which is fitted to slide 

 accurately in adjustable bearings in the body-tube B. The nose- 

 piece tube has a short projecting arm C, by means of which it is 

 pressed upwards by a strong spiral spring mounted in a cylindrical 



* Argelander used to apply two drops of varnish at each end of his webs. He 

 first fixed each extremity by a drop of shellac varnish, and after that had dried, 

 he applied a drop of copal varnish nearer the centre of the frame ; the latter took 

 a long time to harden, but gav ultimately a much stronger attachment. 



