ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 285 



cement is easily applied. When dry, the slide may be finished with 

 colours prepared from tube paints mixed with benzole balsam, or 

 with dammar and benzole. Before mounting, if a dark background is 

 desired, a disk of asphalt of any desired size turned in the centre of 

 the ring will be found convenient. Over the asphalt a small-sized 

 cover-glass may be used for the object to be placed upon, or the 

 asphalt may be covered with shellac when dry. The object may be 

 fastened with gelatine or gum arable, or made to adhere to the coat of 

 shellac before it becomes dry. 



Bauscli and Lomb Circle Cutter.* — This instrument for cutting 

 circles of thin glass (Fig. 60) is intended to be attached to the 

 turntable, by means of the screw shown at the right of the figure, so 



that the cutting point stands over the turning plate. The thin glass 

 is placed upon the turntable and held by the central pin which then 

 revolves with the glass. A gentle pressure causes the cutting point 

 to touch the glass, and perfect circles can thus be readily obtained. 



Wax and Guttapercha in Dry Mounting. f — Prof. W. A. Eogers, 

 of Harvard College Observatory, writes : — "Notwithstanding the general 

 condemnation of wax as a cement for covers 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 experience in 

 unsuccessful moimtings 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. Whatever 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 characteristic defects of diflerent kinds of 

 mountings. 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 thoroughly a glass slide 



* Amer. Mon. Micr. Jouru., il. (18S1) pp. 225-6 (1 fig.), 

 t Ibid., p. 190. 



