| λ | 7000 | 6000 | 5000 | 4000 |
| 1/λ⁴ | 1 | ·504 | ·260 | ·107 |
Supposing λ7000 by the scattering of small particles loses one-tenth of its luminosity, then λ6000 would have ·454 of its original brightness; λ5000, ·234; and λ4000, ·095; that is, whilst λ7000 would lose one-tenth only of its luminosity, λ4000 in the violet would retain not quite one-hundredth of its brightness.
During the years 1885, 1886, and 1887, the writer measured the luminosity of the solar spectrum at different times of the year, and at different hours of the day (see Phil. Trans. 1887: "Transmission of Sunlight through the Earth's Atmosphere"), and from the results he found that the smallest coefficient of scattering for one atmosphere at sea-level for each wave-length was ·0013, when λ⁻⁴ was for convenience sake multiplied by 10¹⁷ (thus λ6000⁻⁴ on this scale was 77·2), and that the mean was ·0017.
| Line. | Wave-length. | 1 λ⁻⁴ × 10¹⁷ | Light after passing through atmospheres of the following thicknesses. | |||||||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 32 | |||
| A | 7594 | 30 | 1 | ·955 | ·908 | ·857 | ·815 | ·775 | ·736 | ·707 | ·665 | ·107 |
| B | 6867 | 45 | 1 | ·926 | ·858 | ·795 | ·735 | ·684 | ·632 | ·583 | ·542 | ·086 |
| C | 6562 | 54 | 1 | ·912 | ·832 | ·759 | ·693 | ·632 | ·576 | ·526 | ·480 | ·019 |
| D | 5892 | 83 | 1 | ·868 | ·754 | ·655 | ·569 | ·494 | ·428 | ·372 | ·323 | ·001 |
| E | 5269 | 129 | 1 | ·803 | ·644 | ·518 | ·427 | ·334 | ·268 | ·216 | ·173 | — |
| F | 4861 | 179 | 1 | ·738 | ·544 | ·402 | ·296 | ·219 | ·161 | ·119 | ·088 | — |
| G | 4307 | 291 | 1 | ·609 | ·367 | ·220 | ·137 | ·084 | ·051 | ·031 | ·019 | — |
| H | 3968 | 403 | 1 | ·506 | ·254 | ·128 | ·071 | ·033 | ·016 | ·008 | ·004 | — |
The following table shows the loss of light for the rays denoted by the principal lines given at page 26, using this last coefficient for different air thicknesses. This is equivalent to giving the intensity of the rays of sunlight when the sun is at different altitudes.
The sun traverses the following thicknesses of atmosphere when it is at the angles shown above the horizon.
| 1 | atmosphere | 90° |
| 2 | " | 30° |
| 3 | " | 19·30 |
| 4 | " | 14·30 |
| 5 | " | 11·30 |
| 6 | " | 9·30 |
| 7 | " | 8·30 |
| 8 | " | 7·30 |
Fig. 10.—Absorption of Rays by the Atmosphere.