MOUNTING AND PRESERVING OBJECTS. 217 



mar the beauty, but posssiblj interrupt a clear view of 

 a, very splendid and delicate object. Then, again, if 

 the microscopist prefers to cut and grind his own glass 

 slips, he should be very careful that there are no sand- 

 specks or air-bubbles in the centre of the slide, or of 

 the glass cover : many a good object has been spoiled 

 from neglect of this precaution. A good light by which 

 to work is also highly important. In using the ordinary 

 microscope, the microscopist should keep both eyes open, 

 the practice of closing the eye not in use being 

 injurious to the sight of both. The beginner who is 

 about to purchase a microscope, will do well to procure 

 a binocular, the price of which has been reduced so 

 much as to bring it within the reach of those of even 

 moderate means. 



In mounting objects in fluid, the glass cover should 

 come nearly, but not quite, to the edge of the cell ; a 

 slight margin being left for the cement, which ought 

 to project slightly over the edge of the cover, in order 

 to unite it securely to the cell. 



To preserve and mount diatomaceas in as nearly as 

 possible a natural condition, they should be first 

 well washed in distilled water and mounted in a 

 medium composed of one part of spirits of wine to 

 seven parts of distilled water. The siliceous coverings 

 of the diatoms, however, which show various beautiful 

 forms under the higher powers of the microscope, 

 require more care in preparation. The guano, or in- 

 fusorial earth containing them, should first be washed 

 several times in water till the water is colourless, 

 allowing sufficient time for precipitation between each 

 washing. The deposit must then be put into nitro- 

 hydrochloric acid (equal parts of nitric and hydro- 

 chloric acids), when a violent effervescence will take 

 place. When this has subsided, the whole should be 

 subjected to heat, brought nearly up to the boiling- 

 point, for six or eight hours. The acid must now be 

 carefully poured off, and the precipitate washed in a 

 large quantity of water, allowing some three or four 

 hours between each washing, for the subsidence of 

 Borne of the lighter forms. The sediment must be 



