296 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
with a vacuum chamber (for expansion and contraction) at one side, 
and not interfere with objectives, then I may turn my attention to fluid 
media, but not till that is a success. For auhydrous cennnts I pro- 
ceed as follows : — 
(1) Give the balsam mounts a coat of good pale copal varnish as 
wide as the ring is to be. Good ‘ elastic gear varnish ’ is so tenacious 
and elastic that I have used polished steel tools (with only one coat 
of it) for over twenty years, it effectually protecting them from rust. 
(2) .Revolve the slide on the turntable and scrape or rub and polish 
the surface a little before applying the second coat ; unless this is done 
the air or gloss will cause some trouble before the next coat will adhere 
to the surface. This operation requires care and skill. 
(3) Build up or fill up the ring around the cover-glass, or, for a cell, 
use the same varnish with a little white beeswax dissolved with it, and 
thinned with turpentine or benzol if too thick. Put on thin coats and 
give each coat plenty of time to dry and harden, so that it may be 
scraped or polished ; it requires very little friction on the surface to 
make the next coat adhere, I prefer a small sharp chisel, which can he 
made cf a bradawl or selected from engravers’ tools. I have some- 
times used a small stick with its end properly dressed and dipped in 
cold water and pulverized pumice-stone, which is then washed away, 
hut I like the chisel best. 
(4) Apply one or two coats of ivory-black mixed with a little varnish ; 
when dry and hard, polish with a scraper, cold water and pumice stone, 
or any suitable polishing material. 
(5) Wash with cold water and a soft brush ; wipe dry with soft 
chamois skin or linen rag. 
(6) Apply an even coat of good amber or copal varnish. I find 
good copal varnish, called ‘ elastic gear varnish,’ and used on carriage 
gearing, better than any gold size. It has to go through mud, rain, 
sand, the burning sun, expansion of heat and contraction of cold, on 
carriages and railroad passenger cars. The body of the ring should be 
hard, solid, tough, elastic, and above all, devoid of brittleness ; it should 
be built up to the top of the cover-glass and finished with the best 
amber or copal varnish. 
To make a colourless copal varnish, select the palest lumps of 
copal gum and crush them into small pieces, but do not pulverize 
when full of dirt ; tie in a bag of fine muslin, and suspend in a wide- 
mouthed bottle of sulphuric ether, when the copal will gradually ooze 
out into the ether. When the gum has been digested, let the bag 
drain off and be thrown into another bottle of ether, which will remove 
all the available gum. It is a good plan to have pleuty of the gum 
so that the liquid will form a varnish sufficiently thick. Then add 
oil of caraway or any slow drying essential oil, as oil of anise, or 
poppy, or sweet almonds, which are as colourless as possible in such 
small quantities; this will make the varnish dry more slowly and 
render it more elastic. When it dries properly, yet is too thick, 
add oil of rosemary or some such colourless essential oil that it may 
dry about as fast as it may be required. If it should dry too slowly 
add more ether and mix thoroughly. 
Some of the essential oils, although colourless, havo slightly 
