This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Actinometer. As there is no outline visible until the tissue is developed, it is useless to examine the carbon tissue when placed upon the negative to print. You must, therefore, be guided entirely by what is known as the actinometer, which is an instrument or contrivance for measuring the printing quality of light, and in this instance is used for determining the exposure necessary for the carbon print. There are many of these instruments on the market, yet none are as good as the one you can make yourself. To make an actinometer, proceed as follows:
146. Provide yourself with a 5x7 clear glass; cover this glass full to the edge with one thickness of fine white tissue or onion skin. The difference between onion skin and tissue paper is merely the quality; the former is of finer grain than the latter. Attach a sheet of this onion skin to the entire plate, by pasting the upper and lower edges; then cut a second sheet the same width, but one inch shorter in length than the first one; add a third sheet one-half inch shorter than the second, and a fourth one-half inch shorter than the third, and so on until you have twelve steps, or twelve sheets, each one-half inch shorter than the other. Through the center of these sheets, beginning at the top, number each step or sheet from one to twelve, the numbers indicating the number of thicknesses of tissue paper over the glass. This constitutes your actinometer, and is the instrument by which to judge the density of the plate.
147. Hold the plate which is covered with the tissue, and is now an actinometer, up to the light in one hand, and in the other hold the negative from which you expect to print. Examine them carefully by locking through them to the light, and notice which one of .these steps supplies the same density as the highest light in the negative from which you expect to print. By the highest light in the portrait negative we mean the most dense portions of the face, which is generally on the forehead. This point of the light or density of the negative must be equal to the thickness of tissue on the actinometer.
Proofing Negative. If this is hard for you to judge at first, place your actinometer in a printing-frame, and the negative from which you expect to print in a second printing-frame. Upon this negative place a piece of ordinary printing-out paper, and on the actinometer use a narrow strip of this same paper sufficient to cover the numbers, but not over one inch wide. After placing the strip on your actinometer, clamp the cover on the printing-frame. It is now ready for printing. As this strip of paper prints, it registers the numbers corresponding to the different depths of printing.
149. Place the actinometer, which is in one printing-frame, and the negative, which is in another frame, in the light to print at the same time. Examine the print on the negative from time to time, and when the high-lights or strongest portions of the negative are merely tinted-not fully printed (the drapery may be well outlined, but the high-lights must be only slightly tinted)-at once remove the actinometer from the light, as well as the frame containing the negative, and compare the two tints. The strip of paper on the actinometer will be printed in steps, one step darker than the other. Compare this proof from the actinometer with the print from the negative, and notice which number corresponds with the faint tint on the strongest high-light on the print from the negative. Note the number of this tint. This is your key and this is the number you must go by when printing your carbon tissue.
150. It is well to register a number corresponding to the actinometer upon the negative from which you are printing. This will save you testing the density of your negative each time you desire to print from this plate. All you will need to do when again printing from such a negative is to note the number written thereon and print accordingly. You are now ready to place the carbon tissue upon the negative you are to print from. Place the tissue in the printing-frame, backing it up with two thicknesses of dry blotting paper. This will bring the tissue in perfect contact with the negative. A printing-frame with good strong springs is necessary for carbon printing. Clamp the back onto your negative firmly. The placing of the carbon tissue on the negative must be done in a room that is free from direct light, as the tissue when dry is very sensitive. Usually a dark-room with a gas or electric light, or ordinary incandescent light, is the best to handle carbon tissues in.