observed that the plate is not appreciably affected within a certain distance from the active matter, and that this distance increases with the thickness of the screen. This distance is obviously equal to twice the radius of curvature of the path of the rays, which are just able to produce an impression through the screen.
These experiments show very clearly that the most deviable rays are those most readily absorbed by matter. By observations of this kind Becquerel has determined approximately the inferior limit of the value of HR for rays which are transmitted through different thicknesses of matter.
The results are given in the table below:
| Substance | Thickness in mms. | Inferior limit of HR for transmitted rays |
|---|---|---|
| Black paper | 0·065 | 650 |
| Aluminium | 0·010 | 350 |
| “ | 0·100 | 1000 |
| ” | 0·200 | 1480 |
| Mica | 0·025 | 520 |
| Glass | 0·155 | 1130 |
| Platinum | 0·030 | 1310 |
| Copper | 0·085 | 1740 |
| Lead | 0·130 | 2610 |
If e / m is a constant for all the rays, the value of HR is proportional to the velocity of the rays, and it follows from the table that the velocity of the rays which just produce an effect on the plate through ·13 mms. of lead is about 7 times that of the rays which just produce an impression through ·01 mm. of aluminium. It will be shown, however, in section 82, that e / m is not a constant for all speeds, but decreases with increase of velocity of