AND HOW TO USE IT. 27 



bubble appears as a bright centre, surrounded by several dark 

 rings of various depths ; by alteration of the focus, these rings will 

 change. With reflected light, a reversion takes place, the centre 

 becoming dark and the periphery light. 



5. — Fat-globules. — Mount a small drop of milk and compare 

 the fat-globules with No. 4. Notice their sharp definition and 

 variation of size. Pass in a little sodium hydrate, and they imme- 

 diately run together. . 



Objects which have been mounted in glycerine or any 

 media liable to evaporation, require the cover-glass to be sur- 

 rounded and slightly overlapped by some cementing fluid, which 

 will gradually dry and form a firm ring. For this purpose, zinc- 

 white varnish, or the brown cement (prepared by E. Ward, of 

 Manchester) are good. 



These cements are best appfied by centering the slip upon a 

 turn-table, fixing the slide down wath the clips, and while rapidly 

 turning to gently bring a camel-hair pencil holding the cement 

 down to the edge of the cover-glass. The object must be care- 

 fully centered and the slide firmly clamped, otherwise the rapid 

 rotation will throw it off the turn-table. If square covers are used, 

 the cement may be applied by gently painting it along each edge. 



Dry Mounting.— The easiest objects that a learner can begin 

 vrith are those that are merely mounted dry in a shallow cement- 

 cell, such as wings and the scales removed from the wings of 

 butterflies, delicate pollen-grains, fern-spores, fish-scales, cuticles, 

 etc. In commencing work, it will be found advisable to prepare 

 a number of slides by putting them on a turn-table, and with a 

 brush charged with brown cement (Ward's), or some similar viscid 

 preparation, the solvent of which should be spirit, run circles upon 

 a number of slides, and put on one side to dry ; those, with brown 

 cement, will, to a certain extent, dry quickly, when, if the cell is 

 not deep enough, another and yet other layers may be added. 

 Now, the slides being ready, suppose we wish to mount a slide of 

 scales from the wing of a butterfly. Cut a piece of ordinary print- 

 ing paper, about i inch by \ inch ; fold it in the middle, and then, 

 placing in the fold a small piece of the wing, press steadily, and 

 with a slight sliding motion, when the scales from both sides will 



