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The Barnet Book of Photography A Collection of Practical Articles

That in general use is known as Johnson's Actinometer


THE ACTINOMETER.

An actinometer must be used to gauge the amount of exposure, as only a faint image, and in some tissues none at all, is visible during or after exposure. The simplest form of instrument is the best. That in general use is known as Johnson's Actinometer, a square tin box containing a long strip of sensitive albumen paper, and provided with a glass lid painted to the colour of printed albumen paper, an opening in the paint in the form of a slit three-sixteenths of an inch in width, from which the paint has been removed. The strip of sensitive paper is made to pass between the top of an inner lid and the painted side of the glass lid underneath the clear slit with the end of the strip protruding at one side of the box. On exposure to light the sensitive silver paper gradually discolours until it closely resembles the colour of the paint, that is called one tint; the tint is changed by pulling the slip forward just the width of the slit, and so on until the requisite number of tints have been printed for the strongest or densest negative in the batch exposed, those negatives requiring less exposure are turned down or removed when the requisite number of tints are registered in each case.

EXPOSURE.

For double transfer from opal the materials required are opal plates, sensitive tissue, French chalk, collodion, double transfer paper, pressure frame, flat camel-hair

brush, chamois leather. Before placing the negative in pressure frame, carefully clean from both sides all finger marks, etc., with the leather, place negative in frame on a paper mask, or provided with a safe edge. After exposure to light, remove from frame and develop on plate prepared as follows:--Rub the whole surface with French chalk on a pad of muslin, afterwards removing loose particles by gentle brushing. Coat with collodion made as follows:--Enamel collodion, 1 part; ether, 1 part; alcohol, 1 part. Filter and coat by pouring a pool on centre of the plate, and, by tilting it, force the collodion to flow into the top right corner, then to the left, then to bottom left, and finally drain off at bottom right corner, rocking the plate the while. The collodion must be allowed to set until it will bear the gentle pressure of a finger in its thickest part, but must not be permitted to dry in any part before plunging into clean cold water to remove the solvents by washing. The time required in washing is variable according to time of year. When the collodion ceases to repel water it is ready to receive the printed tissue. Soak the tissue for the requisite time, but not so long as to become quite saturated, bring it into contact with collodionized side of plate, remove to squeegeeing board, place over it a piece of wet rubber cloth, or a piece of wet thick single transfer paper, coated side up, to prevent injury to exposed margin of collodion and to facilitate the smooth passage of squeegee over the surface in removing excess of water. If, on removing the covering from the plate, the back of tissue is found to be unevenly wet, blot or place plate in a rack to drain; in a few minutes develop in warm water, temperature 90 deg. to 100 deg. F. Be careful to remove the backing paper _under water_, _and as soon as possible after immersion_ in the warm bath. Finish development by laving or pouring warm water over the print from jug or other vessel, until all details are brought out. When washing is finished the print should look rather light, as in drying a decided increase in strength is obtained; rinse _slightly_ in alum solution to stop bleeding only, place in clean cold water to wash out any remains of bichromate, thoroughly rinse by dashing water upon the print to remove any particles of solid matter that may have stuck to its surface; place in a rack to dry, and transfer as soon after drying as possible. The transfer paper is cut a trifle larger than the net size of the print, but less than the opal support; it is soaked in warm water until the surface is slimy to the touch, but not soft enough to break under pressure between finger and thumb. The softened transfer paper is placed in clean cold water into which the dry print is plunged, water dashed upon its surface to remove air; the two surfaces are brought into contact under water, and squeegeed into contact as in first mounting before development. When thoroughly dry, the print may be removed from the opal plate by inserting the point of a knife at the edge.


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