MICROSCOPES. 249 



surface, and when a high power is used, the aniraalcula 

 are continually getting out of the focus by diving to the 

 bottom, and the drop very soon dries up. 



The following contrivance effectually removes these ob- 

 stacles when the focus of the power in use will admit one of 

 the pieces of glass to intervene between the surface of the 

 lens and the object. It consists of two pieces of glass fixed 

 with a small portion of sealing-wax between them, and left 

 open at the top. For viewing animalcula, these pieces 

 should be pressed as close together as possible, but for 

 larger insects they may be left more open. If a single drop 

 of water is taken up by the small spoon at the end of the 

 forceps, and spread over the orifice, it will run in be- 

 tween the two glasses, by which means the surface of the 

 water is rendered perfectly even, and the animalcula are 

 distributed more truly, and confined in a more limited space 

 whereby their forms and movements are more discernible 

 This plan also prevents evaporation from the surface, which 

 often dims the lens and perplexes the observer; a single 

 drop of water may be kept for hours in this way, with any 

 curious animalcula alive in it. The glasses can be cleaned 

 out by introducing a piece of thin writing paper between 

 them, and a drop of pure water. By leaving sufficient room 

 between the glasses, the larger aquatic insects may be 

 viewed ; such as small tadpoles, the ephemera, and the 

 water flea. When animalcula and other minute objects 

 are viewed with a lens of short focus, the following con- 

 trivance is simple and useful ; upon a suitable piece of glass 

 describe a circle with white paint ; when dry, place the drop 

 of water within the circle, and cover it with a very thin 

 piece of talc, the space between the talc and the glass, pro- 

 duced by the paint, affords sufficient room for the animal- 

 cula to move, the talc (if sufficiently thin) offers no obstruc- 

 tion in bringing the object to the focus of the lens. 



Slides for Transparent and Opaque Objects. You will 

 find clear slips preferable to talc in forming the slides for 

 transparent objects ; they may be prepared in the following 

 manner : take two slips of glass about the size of the ivory 

 slides ; then get a piece of writing paper with holes of the 

 same size ; wet one side of the paper with gum water, and 

 lay the glass upon it, suffering it to dry ; then place your 

 objects in the holes, wet the other side in the same manner, 



