174 THE AMERICAN MONTHLY [September, 
Any lettering or numbering of parts, or a written title, may be executed 
upon the dry gelatin film with india ink by means of a steel pen, before cov- 
ering or varnishing. 
The best coloring matter is carbon, and for black the best india ink is su- 
perior to any other substance. For tones bordering on black the following 
are good formule :— 
Pure black.—Dry india ink, 1 part to each 30 parts gelatin. 
Warm black.—India ink, 15 parts; vandyke brown, 2 parts: venetian red, 
2 parts; and of the mixture 1 part to 30 gelatin. 
Deep black.—India ink, 20 parts; indigo, 2 parts; carmine lake, 1 part; 
and of the mixture, I part to 35 gelatin. 
Purple black.—India ink, 5 parts; indian red, 5 parts; burnt umber, 2 
parts ; indigo, 1 part; and use 1 part to 30 gelatin. 
Pure levigated lamp-black alone may be used in many cases. 
The printed tissue gains intensity after exposure, and hence must be devel- 
oped soon after exposure. The following facts should be borne in mind, to 
guide the various steps of the process. The bichromate bath should be 
weaker in warm weather or temperature than in cold, and should ordinarily 
not be used at a temperature above 60° F. and 4o° F. is better. The stronger 
the bath the softer the prints and more sensitive the tissue, but too strong a 
bath will ruin the tissue, or even dissolve it. The sensitized tissue should 
be dried quickly, as slowly dried tissue is not equally sensitive in all portions, 
but if too quickly dried the tissue may crack. 
Although the above directions may seem voluminous, the actual processes 
are speedily and easily carried out, the control of the operator over the char- 
acter of the picture, and the cheapness, certainty and permanence of the re- 
sult, are not equalled by any other photographic process. In Europe the car- 
bon process for portraiture, copying, enlargements, etc., is much more ex- 
tensively followed than the silver process almost exclusively in vogue in 
America. 
Sample prints on paper, and lantern slides, by the process above described 
are forwarded to the ,/owrza/, and may be seen by. those interested in the 
subject. 
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Warm stages.—The Yournal of the Royal Microscopical Soctety in 
the April number contains an article which occupies 18 pages of that Journal 
upon warm and cold stages. It contains 35 illustrations, and is an exhaust- 
ive statement of the various mechanisms in use to control the temperature 
of an object under microscopic examination. 
Four different principles of heating have been adopted, viz. :—(1) hot air ; 
(2) electricity; (3) conduction through metal plates, and (4) warm water. 
Of these one form of (3) is perhaps most convenient for general use; it may 
be made by bending a ring upon the end of a rod of copper wire four inches, 
and then hammering the ring very thin and flat. The ring has a central 
opening of 4 inch diameter. The ring is placed between the stage and the 
side, and light passes through it upon the object. The end of the rod is 
placed in the flame of a spirit-lamp. The temperature may be conveniently 
regulated by a bit of paraffine of known melting point placed upon the slide. 
For warming slide to stimulate amceboid movements of corpuscles, or the 
movement of sluggish amceba, this device will be found entirely adequate. 
For exact studies such simple devices will not suffice, and for these the 
thermocaustic stage of Dr. Dallinger or Stricker’s constant and varying tem- 
perature apparatus become necessary. 
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