190 



THE AMEEICAN MONTHLY 



[October, 



Note on the Use of Wax in Dry 

 Mounting. 



Notwithstanding the general con- 

 demnation of wax as a cement for co- 

 vers in dry mountings, it is doubtful 

 whether the objections urged against 

 its use, are altogether valid. I have 

 had rather more than my share of ex- 

 perience in unsuccessful mountings 

 of this class. During the past five or 

 six years, I have been engaged upon 

 the problem of the exact subdivision of 

 any given unit into equal parts. What- 

 ever success I may have gained in 

 this direction has, I suspect, been 

 somewhat more than counterbalanced 

 by the deterioration of the ruled plates 

 through the condensations which 

 have formed under the covers. 



I have lately collected quite a 

 large number of these plates for the 

 purpose of studying the characteris- 

 tic defects of different kinds of moun- 

 tings. As the result of this study, I 

 have reached the conclusion that, for 

 the most part, the primary cause of 

 the condensations which form under 

 the covers, is the moisture remaining 

 upon the glass after the operation of 

 mounting. No matter how thorough- 

 ly a glass slide may be rubbed, if it 

 is immediately held over a flame a 

 certain amount of moisture will ap- 

 pear. 



The evaporation from certain 

 kinds of cement, without doubt ag- 

 gravates the difficulty, and probably 

 this is, in some cases, the indepen- 

 dent cause of " sweating." 



Nearly all of the slides examined 

 were prepared in the following way : 

 First, the cover-glass being held in 

 position upon the slide by a clip, the 

 moisture was expelled by heating. 

 After the glass had become sufficiently 

 cooled, small bits of white wax were 

 placed around the edge of the cover- 

 glass. The blunt point of a heated 

 piece of metal was then passed slowly 

 around the cover, and the melted 

 wax flowed under it, far enough to 

 hold it in position. The larger num- 

 ber of the slides prepared in this way 



were found to be well preserved. 

 When, however, rings of cement were 

 turned upon the slides, the protection 

 was in almost every case less perfect. 

 In every case in which shellac with 

 anilin coloring was used, condensa- 

 tions on the under side of the cover- 

 glass were found. The covers of se- 

 veral slides were removed, and in no 

 case was there any sweating found 

 upon the surface of the slide. 



About eighteen months ago, my 

 attention was called to the use of 

 sheet gutta-percha rings for dry moun- 

 ting. My first experience with these 

 rings was not altogether satisfactory. 

 It is now evident that I did not, at 

 first, apply sufficient heat to expel all 

 of the moisture between the cover 

 and the slide. 



After an experience of several 

 months, I am convinced that slides 

 prepared in the following way, will 

 remain in a perfect state of preserva- 

 tion for any length of time. Use 

 gutta-percha rings having a thick- 

 ness of about one five-hundreth of an 

 inch, and a diameter about one twen- 

 tieth of an inch less than that of the 

 cover-glass. Hold the cover in posi- 

 tion, upon the ring with a light clip 

 while the gutta-percha is being melted 

 by a gentle heat. If too much heat is 

 applied at first, the ring will lose its 

 normal shape. After the gutta percha 

 is thoroughly melted, the slide should 

 be heated sufficiently to expel every 

 particle of moisture from under the 

 cover. While the slide is hot apply 

 white wax to the surface, the melted 

 wax will run under the cover and will 

 be stopped by the ring. After cover- 

 ing, the wax can be removed from 

 the surface of the glass with turpen- 

 tine. 



I shall esteem it a favor to be in- 

 formed of any case in which a ruled 

 plate mounted in this way, has failed 

 to remain in good condition. 



Wm. a. Rogers. 



Harvard College 



Observatory. 



Sept. gth, 1881. 



