used once, and after use is easily removed and the one under it exposed ready for use.
In the carbon printing process the instrument is exposed to the light with the negative which is to be printed from, and when it shows six degrees, the first quarter of the negative is covered with black paper between the negative and the carbon tissue, or otherwise; when eight, the second; when ten, the third; and when twelve, the fourth. In this manner the single parts to 6, 8, 10, and 12, have been printed. The print is then developed, and notice taken of which part shows the best intensity, i.e., 6, 8, 10, or 12, and ever after, the time of exposure for that negative in the same light is established. When examining the scale it should be held to the light, and the eyes should be shielded from bright light with the hand or otherwise. № 2 will appear first, and the others with decreasing distinctness. When examining the scale hold it about eight inches from the lamp, allowing a bright light to shine perpendicularly upon the paper, and examine the latter obliquely, keeping the eyes from the flame in the direction of the figure 25.
The scale is also provided with hands and letters (omitted in our drawing), and to which attention should also be given, as they facilitate observation. Dr. Vogel does not take the highest figure visible as the copying grade, but the next one below it. For Rowell's tissue he finds for a medium negative that 11 or 12 is the proper degree, or for a dense negative 14 or 15; for Swan's, 15 ; and that it is better to take a degree more than less, i.e., better to over-expose than to do the reverse. The Photometer is equally useful in the photo-lithographic, photo-engraving, enamel, and aniline processes.
For determining the time necessary for the exposure
