82 W. R. TRAVISS ON AN EXPANDING STOP. 



The lever and slotted plate should be made in one piece, as 

 Fig. 5, with a screw tapped into one plate, tight, and to " clear " 

 in the other. Sufficient play must be allowed to give the sheaves 

 room when closed. The apparatus being now complete, the next 

 process is to black it. This can easily be done as follows : — Put 

 a small piece of copper wire into nitric acid, and let it remain 

 until the solution is a decided green. Add to it a little water. 

 By way of trial, take a bit of clean scrap brass and dip it into 

 the solution ; then pass it to and fro through a small flame (gas 

 or spirit-lamp) until the flame ceases to be coloured green. As 

 the sheaves are very small and thin, care must be taken in the 

 " burning off" not to get them too hot. This done, brush all 

 the parts lightly with an old tooth-brush which has been rubbed 

 on a piece of stove blacklead two or three times. The holes and 

 slots may be cleaned out afterwards with a pointed match. See 

 that the sheaves are flat, and now put the whole thing together 

 for the last time. 



There are many ways in which the sheaves may be blackened. 

 If it is found that the pins become loose by the acid process, I 

 would advise the use of some of the " dull black " varnishes 

 such as are on the market, or dissolve a small piece of shellac in 

 alcohol and mix a little lamp- or gas-black with it, painting it on 

 the sheaves, etc., with a soft camel-hair brush, and then gently 

 warm them. Do not, however, make the black too thick. 



Journ. Quekctt Microscopical Club, Ser. 2, Vol. X., No. 60, April 1907. 



