What has been here proved true of the totality of the ultra-red rays is true for each of them singly. Placing our linear thermo-electric pile in any part of the ultra-red spectrum, it may be proved that a ray once emitted continues to be emitted with increased energy as the temperature is augmented. The platinum spiral so often referred to being raised to whiteness by an electric current, a brilliant spectrum was formed from its light. A linear thermo-electric pile was placed in the region of obscure rays beyond the red, and by diminishing the current the spiral was reduced to a low temperature. It was then caused to pass through various degrees of darkness and incandescence, with the following results:—
| Appearance of spiral. | Energy of obscure rays. | ||
| Dark | 1 | ||
| Dark | 6 | ||
| Faint red | 10 | ||
| Dull red | 13 | ||
| Red | 18 | ||
| Full red | 27 | ||
| Orange | 60 | ||
| Yellow | 93 | ||
| Full white | 122 | ||
Here, as in the former case, the dark and bright radiation reached their maximum together; as the
