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 
E 
