296 Professor Jagadis Chunder Bose [Jan. 29, 



the electric beam in the given direction along narrow tubes, and 

 receive it in another tube in which is placed the receiver. The waves 

 experience great difficulty in passing through narrow apertures, and 

 there are other troubles from the interference of direct and reflected 

 waves. These difficulties were ultimately overcome by making suitable 

 radiators emitting very short waves ; the three radiators here ex- 

 hibited, give rise to waves which are approximately ^ inch, ^ inch 

 and 1 inch in length. The intensity of emitted radiation is mode- 

 rately strong, and this is an advantage in many cases. It sometimes 

 becomes necessary to have a greater intensity without the attendant 

 trouble inseparable from too long waves. I have been able to secure 

 this by making a new radiator, where the oscillatory discharge takes 

 place between two circular plates and an interposed platinum ball. 

 The sparking takes place at right angles to the circular j^lates. The 

 intensity of radiation is by this expedient very greatly increased. 

 The parallel pencil of electric radiation, used in many of the experi- 

 ments to be described below, is only about half an inch in diameter. 

 The production of such a narrow pencil became absolutely necessary 

 for a certain class of investigations. Merely qualitative results for 

 reflection or refraction may no doubt be obtained with gigantic 

 mirrors or prisms, but when we come to study the phenomena of 

 polarisation as exhibited by crystals. Nature imposes a limit, and this 

 limitation of the size of the crystals has to be accepted in conducting 

 any investigation on their polarising properties. 



The greatest drawback, however, in conducting experimental 

 investigations with electric radiation arises from the difficulty of 

 constructing a satisfactory receiver for detecting these waves. For 

 this purpose I at first used the original form of coherer made of 

 metallic filings as devised by Professor Lodge. It is a very delicate 

 detector for electric radiation, but unfortunately I found its indica- 

 tions often to be extremely capricious. 



The conditions for a satisfactory receiver are the following : — 



(1) Its indications should always be reliable. 



(2) Its sensitiveness should remain fairly uniform during the 

 experiment. 



(3) The sensibility should be capable of variation, to suit difierent 

 experiments. 



(4) The receiver should be of small size, and preferably linear, 

 to enable angular measurements to be taken with accuracy. 



These conditions seemed at first almost impossible to be attained. 

 The coherer sometimes would be so abnormally sensitive that it 

 would react without any apparent cause. At other times, when 

 acting in an admirable manner, the sensitiveness would suddenly 

 disappear at the most tantalising moment. It was a most dreary 

 experience when the radiator and the receiver failed by turns, and it 

 was impossible to find out which was really at fault. 



From a series of experiments carried out to find the causes which 

 may affect prejudicially the action of the receiver, I was led to sup- 



