OF MICROSCOPIC OBJECTS. 49 



me that one of these things has occurred either the india- 

 rubber or the asphaltum has not been pure, or the naphtha 

 has been wood instead of mineral. In the early photo- 

 graphic days every artist made a form of this varnish to 

 use with glass positives, and I never heard a complaint of 

 difficulty. 



This is often used by photographers as a black varnish 

 for glass, and never cracks, whilst it is very adhesive. Dr. 

 Carpenter, however, states that his experience has not been 

 favourable to it ; but I have used it in great quantities and 

 have never found it to leave the glass in a single instance 

 when used in the above proportions. The objections to it 

 are, however, I think easily explained, -when it is known 

 that there are many kinds of pitch, &c., from coal, sold by 

 the name of asphaltum, some of which are worthless in 

 making a microscopic cement. When used for this purpose, 

 the asphaltum must be genuine and of the best quality 

 that can be bought. The above mixture serves a double 

 purpose to unite the cell to the' slide, and also as a "finish- 

 ing " varnish. But it is perhaps more convenient to have 

 two bottles of this cement, one of which is thicker than 

 common varnish, to use for uniting the cell, &c. ; the other 

 liquid enough to flow readily, which may be employed as a 

 surface varnish in finishing the slides. 



The brushes or camel-hair pencils should always be 

 cleaned after use; but with the asphalt varnish above 

 mentioned it is sufficient to wipe off as carefully as possible 

 the superfluous quantity which adheres to the pencil, as, 

 when again used, the varnish will readily soften it ; but, of 

 course, it will be necessary to keep separate brushes for 

 certain purposes. 



Here it may be observed that every object should be 

 labelled with name and any other descriptive item as soon 

 as mounted. There are many little differences in the 

 methods of doing this. Some write with a diamond upon 

 the slide itself; but this has the disadvantage of being not 

 so easily seen. For this reason a small piece of paper is 



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