posed to a radiation of 30° of the thermomultiplier, have furnished the following results:
| Order of the screens. | Deviations of the galvanometer. | |||
| 1. | Translucid | 5°·38 | ||
| 2. | 6·50 | |||
| 3. | 8·66 | |||
| 4. | Dull | 12·58 | ||
| 5. | 14·79 | |||
| 6. | Slightly dull | 17·42 | ||
| 7. | Transparent | 18·79 | ||
| 8. | 19·15 | |||
These transmissions present nothing extraordinary: the quantity of heat which passes through the medium is greater in proportion as the surface is more finely polished, as it happens in respect to light. The only thing to be remarked is, that in the high degrees of polish a slight difference produces a very slight effect. This is evident from the observations made on Nos. 7 and 8.
Similar processes enable us to determine the influence of thickness, which is one of the elements most necessary to be known in the theory of transmission.
Four pieces cut out of a fine mirror were reduced with great nicety to different degrees of thickness in the ratio of 1, 2, 3, 4: particular care was taken to give to their principal surfaces a perfect parallelism, and the highest polish possible. The following are the deviations which they successively produced in the index of the galvanometer under the action of the same radiation, namely 30°:
| Thickness of the screens in millimetres. |
Deviations of the galvanometer. |
Corresponding forces. |
| 2·068 | 21°·625 | 21·850 |
| 4·136 | 20·312 | 20·343 |
| 6·202 | 19·687 | 19·687 |
| 8·272 | 19·375 | 19·375 |
Each number in the second column is deduced from fifteen observations: the quantities registered under the denomination of forces, representing in this particular case the respective temperatures or quantities of rays transmitted, have been calculated according to the principles with the exposition of which we concluded our general observations. The force or temperature answering to 30°, as given by the table of intensities, is 35·3; now, by dividing each number of the third column by 35·3, we shall obtain the ratios of the transmitted rays to the incident rays. The difference between each of these quotients and unity will give the corresponding loss; that is, the proportional part of
