406 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



for rapid work it offers many facilities. In addition to pencils of 

 the usual kind, some with broad leads will be found useful for 

 covering larger surfaces. Very delicate tints can be made with 

 blacklead powder rubbed on the paper with a suitable leather stump. 

 Tints of any depth can also be obtained from blacklead used as a 

 water-colour, which can be procured in cakes. 



Blacklead, charcoal, and chalk drawings can be permanently fixed, 

 by saturating the paper from behind with a varnish composed of 

 bleached shellac and alcohol. This should be very freely applied 

 and dried in a warm room or with caution before a fire. The strength 

 should be such that it will just dry without leaving a gloss on the 

 paper. Wiusor and Newton's white lac varnish, mixed with an 

 equal bulk of methylated spirit, will be the right strength. After 

 this treatment a pencil drawing may be placed in the portfolio, and 

 even exposed to some amount of rubbing, without injury. The var- 

 nish does no harm to any water-colour tints that may be used in 

 ccmibinatiou with pencil. 



Ulmer's Silk Thread Movement.* — J. Ulmer suggests the use 

 of a silk thread for microscope-tubes and the eye-pieces of telescopes. 



The tube T (Figs. 69-72) has above and below in the socket two 

 guides c c, against which it is gently pressed by the small pulley d and 

 spring e, by which means easy sliding is secured. The movement of 



Fig. 69. 



Fig. 71. 



the tube is effected by the silk thread /which is attached fo a spring g 

 and screw h, both of which are fixed to the tube. The spring is slit as 

 shown in the figures, and the screw is hollow and serves for stretching 

 the thread and the spring, after the former has been laid in the slit, 

 and turned round the pinion i, which is fluted to avoid slipping. The 

 rotation of the tube is prevented by making the support by which the 

 female screw at Ji is attached to the tube slide in a slit in H. 



The apparatus works, it is said, without any " loss of time," and 

 secures an easy motion, at the same time being very simple. 



* Centralztg. f. Optik u. Med., ii. (1881) p. 148 (4 figs.). 



