Wave-length of Electric Radiation ly Di/r action Grating. 175 



It should be remarked here that numerous difficulties were 

 encountered in carrying out the experiments. The reflections from 

 the walls of the room, from the table, &c., were at first sources of 

 considerable trouble. By taking special care, I succeeded in elimi- 

 nating these disturbances. The radiating balls were placed about 

 1 cm. inside the square tube. This prevented the lateral waves 

 acting on the receiver. The receiver was provided with a guard tube, 

 which stopped all but the diffracted radiation reaching the sensitive 

 surface. The insulated wires from the ends of the receiver were 

 protected by thick coatings of tinfoil, and led to the galvanometer, 

 which was placed at a considerable distance. The cell and the 

 galvanometer were enclosed in a metallic case with a narrow slit 

 for the passage of light reflected from the galvanometer. 



In spite of all these precautions, I was baffled for more than six 

 months by some unknown cause of disturbance which I could not 

 for a long time account for. It was only recently, when nearly 

 convinced of the futility of further perseverance, that I discovered 

 the mistake in supposing sheets of tinned iron to be perfectly 

 opaque to electric radiation. The metal box which contains the 

 radiating apparatus seems to transmit a small amount of radia- 

 tion through its walls, and if the receiver happens to be in a very 

 sensitive condition it responds to the feeble transmitted radia- 

 tion. I then made a second metallic cover for the radiating box, 

 which precaution was found effective, provided the receiver was riot 

 brought very close to the radiator. The receiver is still affected if 

 placed immediately above the radiator tube, though two metallic 

 sheets be intervening. For this reason I had to postpone taking the 

 reading for minimum deviation till I had made a radiation-proof 

 box. A soft iron box (to prevent escape of magnetic lines of induc- 

 tion), enclosed in a second enclosure of thick copper, would, I expect, 

 be found impervious to electric radiation. 



With the second protective enclosure, all difficulties were prac- 

 tically removed. As a test for the absence of all disturbing causes, I 

 observed whether the receiver remained unaffected when the grating 

 was " off." There is a further test for the absence of external dis- 

 turbances. The response, if only due to the diffracted beam, depends 

 on the position of the radiator on the focal curve. If this angle of 

 incidence is decreased, there should then be no action on the receiver. 

 I found the positions of the radiator on the focal curve producing 

 action on the receiver, to be well denned, and I experienced no further 

 disturbance due to stray radiations. 



The grating is fixed vertically on the table, so that its centre is at 

 the same height as that of the middle of the receiving and radiating 

 tubes. A small mirror is fixed at the middle of the central strip. 

 The observer, placing his eye at the same height as that of the 



