I have thus confirmed many observations originally made by eye observation with the fluorescent screen. Here the power of X-rays to pass through wood or cardboard becomes useful. The photographic plate can be exposed to the action without removal of the shutter of the dark slide or other protecting case, so that the experiment need not be conducted in darkness. Manifestly, unexposed plates must not be left in their box near the vacuum tube. It seems now questionable whether the impression on the plate is a direct effect of the X-rays, or a secondary result induced by the fluorescence of the material of the plate.
Films can receive the impression as well as ordinary dry plates.
I have not been able to show experimentally that the X-rays give rise to any caloric effects. These, however, may be assumed, for the phenomena of fluorescence show that the X-rays are capable of transformation. It is also certain that all the X-rays falling on a body do not leave it as such. It is clear from the experiments that this is not due to want of permeability on the part of the structures of the eye. For comparison, light rays were allowed to fall on the prism as the apparatus was set up for the experiment.