MOUNTING TRANSPARENT OBJECTS DRY. 221 



it, the other being brought down in its place ; and as the heated 

 plate will be some little time in cooling, the firm attachment of 

 the cells will be secured. If, on the other hand, there be only a 

 single plate, and the operator desire to proceed at once in mount- 

 ing more cells, the slides already completed should be carefully 

 removed from it, and laid upon a wooden surface, the slow con- 

 duction of which will prevent them from cooling too fast. Be- 

 fore they are quite cold, the superfluous glue should be scraped 

 from the glass with a small chisel or awl ; and the surface should 

 then be carefully cleansed with a solution of potash, which may 

 be rubbed upon it with a piece of rag covering a stick shaped 

 like a chisel. The cells should next be washed with a hard brush 

 and soap and water, and may be finally cleansed by rubbing with 

 a little weak spirit and a soft cloth. In cases in which appearance 

 is not of much consequence, and especially in those in which the 

 cell is to be used for mounting large opaque objects, it is de- 

 cidedly preferable not to scrape off the glue too closely round 

 the edges of attachment, as the "hold" is much firmer, and 

 the probability of the penetration of air or fluid much less, if 

 the immediate margin of glue be left, both outside and inside 

 the cell. 



122. Mounting Objects Dry. — There are certain objects, which, 

 even when they are to be viewed by transmitted light, are more 

 advantageously seen when simply laid on glass, than when they 

 are immersed either in fluid or in balsam.' This is the case 

 especially with sections of bones and teeth, much of whose in- 

 ternal structure is obliterated by the penetration of fluid ; and 

 also with the scales of Lepidopterous and other Insects, whose 

 minute surface-markings are far more distinct when thus ex- 

 amined, than when treated in any other way.' For presei'ving 

 such objects, it is of course desirable that they should be pro- 

 tected by a cover ; and this must be so attached to the glass 

 slide, as to keep the object in place, besides being itself secured. 

 For this purpose sealing-wax varnish is often used, but is unsuit- 

 able on account of its brittleness when dry ; gold size mixed with 

 lamp-black is much to be preferred, and, if carefully laid on, will 

 not tend to run in between the cover and the slide. Jf the object 

 have a,ny tendency to curl up, or to keep off the cover from the 

 slide by its own "spring," it will be useful, while applying the 

 varnish, to make use of pressure, such as that afforded by the 

 little implement represented in Fig. 62. This pressure should 



' It is affirmed by two high authorities on all that relates both to the theoretical and 

 practical action of Object-glasses of largfe aperture (namely, Prof, Robinson and Mr. 

 Wenham), that the effect of mounting delicate test-objects in Canada balsam is practi- 

 cally to reduce the aperture, since no rays can diverge after passing through a stratum 

 of this substance, at a greater angle than 85° or 90°. Hence they recommend that the 

 " difficult" DiaiomacecB should not be mounted in balsam, if they are to be viewed by 

 objectives of 120° or 130°. Their position is disputed, however, by Prof. Bailey (U. S.), 

 who affirms that balsam-mounted specimens are preferable as test-objects. Those who 

 are interested in this question, will do well to consult the papers of these gentlemen, in 

 the 2d and 3d volumes of the " Quart. Journ. of Microsc. Science." 



