us emanations from any source,
the fumes of turpentine oil, etc., which, by reducing the chromic salt,
cause the insolubilization of gelatine, prevent the print to adhere on the
support or the clearing of the image, which may even refuse to develop.

The sensitive tissue keeps well for three or four weeks in cool and dry
weather, and no more than eight or ten days in summer unless well
desiccated and kept in a preservative box. If kept too long the image
cannot be developed.

_The Photometer.--_The time of exposure is regulated by means of a
photometer. Of all the photometers which have been devised for that
purpose we do not know any one more practical than that suggested in 1876
by Mr. J. Loeffler, of Staten Island. It is made as follows: On a strip of
a thin glass plate, 6x2 inches, make four or five negatives, 11/2x11/4 inch,
exposing each one exactly for the same period and developing in the usual
manner, but without any intensification whatever. It is even advisable to
reduce the intensity if they were opaque. Fix, etc., and apply a good
hard varnish. Now cover the back of these negatives with strips of
vegetable paper or transparent celluloid, or, better, of thin sheets of
mica, in such a manner as there be one thickness on the second negative,
two on the third, three on the fourth, etc., leaving the first one
uncovered. Then place on the whole a glass plate of the same size as the
first and border like a passe-partout.

_The Negatives.--_For the carbon process the negatives should be intenser
than those intended for printing out on silver paper. However, good
proofs may be obtained from any negatives, so to say, by varying the
strength of the bichromate solution, as, also, by _using the tissue
freshly sensitized for weak negatives,_ in order to obtain vigor, and _for
strong negatives, the tissue two or three days after its preparation,_
when it yields better half tones. Printing dodges are also resorted to.
That the most commonly employed consists to varnish the back of the
negatives with a matt varnish, or to stretch on the same a sheet of
mineral paper upon which the retouches are made by rubbing graphite,
chrome yellow, pink or blue colors to strengthen the shadows or the
whites, as the case requires. As a rule, it is advantageous to cover the
printing frame with tissue paper, whatever be the quality of the
negatives.

The negatives should be bordered with deep yellow or orange-red paper to
form