THE X-RAYS. 147 



this plate is a fluorescent screen which fills almost the whole cross sec- 

 tion of the jar. This screen can receive neither the direct rays nor 

 those a single time diffusely reflected from solid substances, as, for 

 example, the glass walls. Before each experiment the jar is filled with 

 dust-free air. When X-rays are allowed to enter in such a way that 

 they are all received by the lead i^late a there is no fluorescence to be 

 observed at h. When the jar is inclined so that the rays can pass 

 through the space between a and h the fluorescent screen is illuminated 

 over that portion not covered by the plate h. When the jar is con- 

 nected with an air pump the fluorescence becomes weaker the further 

 the exhaustion proceeds, and when air is admitted the intensity again 

 increases. 



I found that no noticeable fluorescence was excited by simple contact 

 with air through which X-rays had shortly before passed, so that, it 

 follows that the air while it is receiving X-rays also sends them out 

 in all directions. 



If our eyes were as sensitive for X-rays as they are for light rays, an 

 actively operating discharge tube would appear to us like a light burn- 

 ing in a room uniformly filled with tobacco smoke. It might perhaps 

 be that the colors of the rays coming direct would be different from 

 those sent out by the air particles. 



The question whether the rays sent out by bodies are the same as 

 those they receive, or, in other words, whether this phenomena is due 

 to diffused reflection or to an action similar to fluorescence, I have not 

 thus far been able to decide. It may be readily shown that the rays 

 coming from air particles are photographically active, and this char- 

 acteristic makes itself manifest in a most unwelcome manner. In order 

 to guard against this effect it is necessary in long exposures to protect 

 the plate by appropriate lead screens. 



2. In order to compare the intensity of the radiation from the dis- 

 charge tubes, and for various purposes, I have made use of a contriv- 

 ance which is constructed similarly to the Bouguer photometer, and 

 which, for the sake of simplicity, I will call a photometer. A rectan- 

 gular piece of sheet lead 35 centimeters high, 150 centimeters long, and 

 0.15 millimeter thick is supported vertically on a piece of board in the 

 center of a long table. On each side of this sheet and movable upon 

 the table stands a discharge tube. At one end of the lead strij) is a 

 fluorescent screen^ so arranged that each half is vertically radiated 

 upon. In the measurements the arrangements were adjusted until the 

 two parts were equally bright. 



Some remarks made upon the use of this instrument may not be out 



'In this and other experiments the Edison fluorescent screen has proved very 

 useful. This consists of a stereoscopic case which can be secured light-tight to the 

 head of the observer, and whose pasteboard bottom is covered with barium-iilatinuni 

 cyanide. Edison employs Scheelite in place of barium-platinum cyanide; but for 

 many reasons I prefer the latter. 



