If sunlight outside the atmosphere be represented by 1 and say 1/10th be cut off by 1 atmosphere, then after transmission through 2 atmospheres only .81 will reach the spectator, and if through 3 only .729. For any atmosphere the diminution will be 1/10th, that is, it will be .9x where x is the number of atmospheres.
If we ascend the factor varies, there are less thicknesses of atmosphere to go through and we get the following table.
| Barometer in Inches. | Visual Transmission Sunlight outside the Atmospheric being 1). | Photographically Actinic Light Transmitted (Sunlight outside the Atmospheric being 1). |
| 30 | .853 | .639 |
| 29 | .866 | .654 |
| 28 | .875 | .672 |
| 27 | .884 | .689 |
| 26 | .891 | .708 |
| 25 | .899 | .730 |
| 24 | .908 | .746 |
| 23 | .915 | .763 |
| 22 | .922 | .787 |
| 21 | .928 | .800 |
| 20 | .934 | .819 |
| 19 | .940 | .833 |
This table and the preceding one will enable a calculation to be made as to the exposure to be given. Thus at sea level with a photographic brightness of sun of 639,000 candles when nearly overhead, it will at 5° above the horizon only have a photographic brightness of about 1000. At about 9000 feet high the photographic brightness would when the sun is overhead be about 800,000 candles, and at 5° it would have a value of 350,000, showing the greater penetration through the thinner atmosphere.
W. de W. Abney, C.B., F.R.S., etc., etc.
Negative Making.
DEVELOPMENT, INTENSIFICATION,
REDUCING, Etc.
When a sensitive plate has been properly exposed under ordinary conditions, there is no visible change. The action of light produces what is known as a latent image or developable image, and in order to convert this into a visible image with sufficient opacity to be useful for printing purposes, it must be developed. In the operation of development, the plate is treated with some solution that will act on the exposed parts of the sensitive film and reduce the silver salts contained therein to metallic silver, in quantity proportional to the amount of light-action, whilst at the same time it produces no appreciable change in those parts of the film on which light has acted the least or not at all, and which correspond to the darkest shadows of the object that has been photographed. The solution used for this purpose is called the developer.
DEVELOPERS—GENERAL.
The substances that can be employed as photographic developers are now somewhat numerous, but the most useful for negative making are pyrogallic acid (also known as pyrogallol, or for brevity as pyro.), ortol, metol, and hydroquinone (also known as quinol). Ferrous oxalate is likewise used in special circumstances, but not for general work. An ordinary developer as mixed for use contains:—
1.—One of the above-mentioned substances (pyrogallic acid, ortol, metol, quinol) which is the actual developing constituent, and is known as