24 PREPARATION AND MOUNTING 



of lamp-black to gold-size and mixing intimately. Dr. Carpenter 

 recommends this as a good finishing varnish, drying quickly and 

 being free from that brittleness which renders some of the others 

 almost worthless ; but it should not be used in the first process 

 when mounting objects in fluid. 



Amongst these different cements and varnishes I worked a long 

 time without coming to any decision as to their comparative 

 qualities, though making innumerable experiments. The harder 

 kinds were continually cracking, and the softer possessed but lit- 

 tle adhesive power. To find hardness and adhesiveness united 

 was my object, and the following possesses these qualities in a 

 great degree : 



India-rubber J drachm. 



Asphaltum 4 oz. 



Mineral naphtha 10 " 



Dissolve the india-rubber in the naphtha, then add the asphaltum 

 if necessary heat must be employed. 



This is often used by photographers as a black varnish for glass, 

 and never cracks, whilst it is very adhesive. Dr. Carpenter, how- 

 ever, 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 " finishing " 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 suf- 

 ficient to wipe off as carefully as possible the superfluous quantity 

 which adheres to the pencil, as, when again made use of, the var- 



