The Microscope. 369 



aqueou8 mounts, while the last, or last two coats, should be as 

 hard as possible ; they will adhere to softer coatings, while they 

 might be too brittle to apply directly to the glass. 



Good gold size has only one fault — it dries too slowly. The 

 best copal varnishes are just as good and dry in much less 

 time. I abhor all fluid mounts, and, therefore, have no use for 

 that miserable, brittle, crumbling white zinc cement which soon 

 assumes a dirty, mud color. 



When glass can be prepared to inclose an object in fluid and 

 be as durable as the cells or tubes of spirit levels, or bulbs of 

 thermometers, with a vacuum chamaer (for expansion and con- 

 traction) 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 anhydrous cements I proceed as follows : 



1. Grive the balsam mounts a coat of good, pale copal var- 

 nish as wide as the ring is to be. Good " Elastic Gear Var- 

 nish " is so tenacious and elastic that I have used polished steel 

 tools (with only one coat of it) for over twenty years, it effect- 

 ually protecting them from rust. 



2. Revolve the slide on the turn table 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 be- 

 fore 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 dis- 

 solved 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 re- 

 quires very little friction on the surface to make the next coat 

 adhere. I prefer a small, sharp chisel, which can be made of a 

 brad awl or selected from engravers' tools. I have sometimes 

 used a small stick with its end properly dressed and dipped in 

 cold water and pulverized pumice stone, which is then washed 

 away, but 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. 



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