The Microscope. 339 



rings, thus applied, are in the way of the lens, which is more or 

 less liable to injury by contact wi:;h the ring of varnish on the 

 covei". I perceive, however, as I look over my collection of 

 slides, that many microscopists never or seldom allow any 

 cement or varnish to rise above the upper surface of cover- 

 glasses. This effectually prevents all trouble with high-power 

 objectives, unless the cover is not parallel with the slide, a very 

 common annoyance, but it does not hold the cover as securely 

 as a thin coat of hard and elastic finishing varnish applied around 

 the edge with a very narrow ring on top, being sure that there 

 is no break between the ring at the edge and that at the top. 



Man}'^ cements have been prepared and are in the market which 

 are very defective in preparation and formula. In a former arti- 

 cle I recommended Winsor and Newton's picture varnish for 

 finishing mounts. It makes a neat finish, but is not a durable 

 coating. All the cements prepared from dammar, mastic, 

 shellac, gum arable, and all the other gums or resins, to my 

 knowledge, (except copal, amber, and a resin or gum nearly as 

 colorless and hard as glass, which resisted fifteen solvents, and 

 for which I have not yet found a solvent nor a name,) all are too 

 soft and brittle, and therefore unfit for cements or varnish. 



White zinc cement, according to my collection of mounts and 

 all that I have purchased, and made myself, is the most defective 

 of all cements. It is not necessary to enter into an argument, or 

 a controversy to prove this question, with any pet theories. As 

 a practical test, " the eating is the proof of the pudding." 



When I examine the slides received from Europe and from 

 every State in the Union, I find that the rings of white zinc 

 shellac, dammar, Brunswick black, marine glue, etc., have pris. 

 matic colors between them and the slide, an evidence that the 

 cement has cracked loose from the glass. Some of my own 

 preparations have also cracked and curled up in the course of 

 time. My slides are in a cool room, seldom heated to 90° P., in 

 the Summer, and never below 40° F., in the Winter, and not 

 exposed to sudden changes of temperature. 



I have resolved to put all cements for my own use to the fol- 

 lowing practical test : spin a ring on a clean slide and let it 

 harden thoroughly, then push a sharp pointed scratch-awl, or a 

 sharp brad-awl through the ring, cutting a groove just wide 

 enough for the tool to pass. This repeated a dozen times on 



