THE MICROSCOPE. 



the light to pass up, and not be put on soHd, as mounts for ordi- 

 nary illuminations are. The object is best attached to the glass 

 circle by means of the gelatine cement, and the slide is to be fin- 

 ished precisely the same as heretofore directed. And finally, all 

 fears of moisture spoiling a beautiful preparation in the future may 

 be dismissed as groundless. 



Our work thus far has been confined to opaque objects requir- 

 ing surface illumination, and it may be said that the great majority 

 of all to be mounted in the dry way are of this class. But many, 

 notably scales and hairs of insects, and plants, many diatoms, sec- 

 tions of pith, etc., are best viewed in the dry state and by 

 transmitted light. Most of these may be mounted upon the 

 cover, (the method of doing so in tlie case of diatoms, 

 having been directed in a former paper), and thus may 

 be mounted in an ordinary cement cell without fear of 

 moisture. The most satisfactory cement for this purpose (and most 

 others), in my experience, is the white zinc, when properly prepared. 

 It dries quickly, has no tendency to run in, and makes a beautiful 

 finish to a mount. It should be used as follows, and the same direc- 

 tions will apply to asphalt or Brunswick black, if those cements be 

 preferred : 



Let us suppose that a 5.8 inch cover is to be used. Placing our 

 glass slip upon the turntable, we proceed to run a ring of cement 

 about its centre, the outer diameter of which shall be slightly in 

 excess of S'a inch, the width of the ring being about Vs of an inch. 

 This first coat must be allowed to harden thoroughly, as on this de- 

 pends all future success of the mount. If the slightest softness is 

 left, it will be sure to yield still further in hot weather, and by 

 capillary attraction run in and spoil the slide. To insure against 

 all possibility of failure, let the slide be set aside for at least 48 

 hours, or else be baked in an oven. When the cover is ready to be 

 place upon it, a fresh ring of the cement is to be run upon the top 

 of the first, extreme care being taken not to let the fresh extend 

 to the inner edge of the old cement, lest it run in by contact be- 

 tween the surfaces of slide and cover. The latter is then to be 

 placed upon the ring center with the forceps, and slightly pressed 

 down. A very thin coating of the cement is now to be applied 

 around the edge, and allowed to harden, after which as many may 

 be applied, with or without colored rings, as the taste of the worker 

 may dictate. 



