232 THE AMERICAN MONTHLY [Au 



even opaque objects require to be properly displayed, 

 whilst objects to be examined with transmitted or direct 

 light — that is, by means of light that passes through the 

 object — require very careful preparation beforehand. 



Slips. — Wooden slips and paper-covered slips are now 

 very rarely used, 3 inch x 1 inch glass slips being now al- 

 most universal. These can be obtained from any optician. 

 They should, preferably, have ground edges, and for gen- 

 eral purposes should be of medium thickness. They will 

 cost from twopence to fivepence per dozen, according to 

 quality, or less for a larger quantity. If any of them 

 should be found to have scratches or specks in the centre, 

 they should be put aside for making opaque mounts. For 

 exceptionally large mounts slips 3 inches x 1£ inch can be 

 obtained. The cover-glasses should be circular, in thick- 

 ness from -006 inch to '008 inch, and might vary in size 

 from | inch to § inch diameter. It would be well to pro- 

 vide oneself with a stock of f inch, £ inch, and $ inch cov- 

 er-glasses, and to note their thickness at the time of pur- 

 chase, and, generally speaking, to adhere afterwards to 

 the same standard for ordinary work. High-power work 

 with objectives of very short focus may require thinner 

 cover-glasses to be used. We would also recommend the 

 purchase of a dozen or so slips with excavated cells of 

 various sizes, i. e. with concavities ground in their centers. 



Before use, all slips and covers must be scrupulously 

 cleaned. It is generally sufficient to wash them with hot 

 water and soap or soda ; but for special work more dras- 

 tic measures may be necessary. The writer generally uses 

 a fairly strong and hot solution of Hudson's Soap, with 

 subsequent careful rinsing and polishing with an old cam- 

 bric handkerchief. The great thing to be avoided is any 

 suspicion of grease, even from the fingers themselves. 

 Cover-glasses must be finally polished with chamois leath- 

 er and as they are very thin and of course easily broken, 

 various contrivances such as buff blocks are obtainable for 



