The American Carbon Manual/Vogel's New Photometer
VOGEL'S NEW PHOTOMETER.
The photometer or actinometer used by Mr. Swan, and described above, was found to be wanting some of the advantages required, but which are entirely secured by Dr. H. Vogel's admirable invention described below.
The objects of this ingenious device is to enable the operator to judge of the proper time to expose a negative in the carbon printing process, and to determine the exposure when making negatives in the glass-room. Those who have experimented in carbon printing know the difficulty of determining the proper exposure, too short or too long a time destroying the print altogether. The use of this little instrument overcomes that. In making negatives on days when the light is variable, difficulty is often experienced in securing the proper time of exposure. This is also the case when making copies, views of interiors, and landscapes. A proper understanding and use of Dr. Vogel's photometer will be found of immense advantage in such cases. The length of exposure of any desired picture, in any desired weather, can at once be ascertained. Mr. Swan has adopted this photometer.
We shall now proceed to describe it.
It consists of a box A A, provided with a lid B B, as shown in the drawing. The lid consists of a frame, a,
by which a glass plate, b, is held; on the upper side of this glass plate is secured a series of thin strips of paper, which are arranged mathematically in layers; each lower layer projecting, like steps, beyond the layer above. This step system thus produced, represents a semi-transparent medium, the transparency of which decreases by degrees towards the thicker end. Black figures upon the under side of the lower strip indicate the number of layers arranged above each such figure. The whole cover, with the paper system in it, can be folded down and fastened by the small hook, c, and is provided with a second cover of wood, to protect the glass from injury, when not in use, and to make and close the exposure.
Within the box is a sliding false bottom, D, which is, by means of a steel spring, pressed upwards against the scale described above, when the lid is closed. Upon this false bottom a number of strips of paper are placed, which have been sensitized by immersion in a saturated solution of bichromate of potash of 1 oz. bichromate to 30 oz. water.
In order to get these in place, the bottom of the box is opened, the spring removed, the false bottom taken out (and may be used as a guide to cut the strips of sensitized paper), the strips placed in the box, the false bottom dropped in upon them, the bottom closed, and the photometer is ready for use.
This must be done in the dark-room, and the fingers should be dry. Strips sensitized in this way will keep a month.
It will now be seen that when the apparatus is exposed to light that the sensitized strip changes color in proportion to the amount of light it receives, the most light passing through the spot marked 2, the next 4, the next 6, and so on, and the change will be rapid or slow, according to the intensity of the chemical action of the light. The black figures admit no light through them, and after exposure appear on the sensitized strip as light figures on a dark ground. A strip can only be 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 of the negative in the camera by the collodion process, Dr. Vogel gives the following directions:
While preparation is being made to take the picture, expose a plate to the light at the place where the model is to sit, one minute, timing it with the watch. Now expose the photometer for one minute in the same light, and note the degree indicated on the sensitized slip.
We believe this instrument will be found useful to every photographer, and, as we understand, it will be furnished at a low price; no doubt all who desire to progress and improve will possess themselves of one or more. Keep the glass clean above the scale. Don't touch the scale with damp fingers.