| Altitude. | Atmosphere. | ||||
| 90° | 1.000 | ||||
| 80° | 1.015 | ||||
| 70° | 1.064 | ||||
| 60° | 1.155 | ||||
| 50° | 1.305 | ||||
| 40° | 1.555 | ||||
| 30° | 1.995 | ||||
| 20° | 2.904 | ||||
| 15° | 3.809 | ||||
| 10° | 5.571 | ||||
| 5° | 10.216 | ||||
| 4° | 12.151 | ||||
| 2° | 18.882 | ||||
| 0° | 35.503 | ||||
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.