298 Professor Jagadis Chunder Bose [Jan. 29, 



face, there is a sudden diminution of the resistance, and the galvano- 

 meter spot is violently deflected. 



By means of a very fine screw the upper sliding piece can be 

 gently pushed in or out. In this way the spirals may be very 

 gradually compressed, and the resistance of the receiver diminished. 

 The galvanometer spot can thus easily be brought to any convenient 

 position on the scale. "When electric radiation falls on the sensitive 

 surface the spot is deflected. By a slight unscrewing the resistance 

 is increased, and the spot made to return to its old position. The 

 receiver is thus re-sensitised for the next experiment. 



The receiver thus constructed is perfectly reliable ; the sensibility 

 can be widely varied to suit different experiments, and this sensibility 

 maintained fairly uniform. When necessary, the sensitiveness can 

 be exalted to almost any extent, and it is thus possible to carry out 

 some of the most delicate experiments (specially on polarisation) with 

 certainty. 



The main difficulties being thus removed, I attempted to construct 

 a complete electric wave apparatus, which would be portable, with 

 which all the experiments on electric radiation could be carried out 

 with almost as great an ease and certainty as corresponding experi- 

 ments on light, and which would enable one to obtain even quantita- 

 tive results with fair accuracy. 



The complete apparatus is here exhibited ; all its different parts, 

 including the galvanometer, and all the accessories for reflection, 

 refraction, polarisation, and other experiments, are contained in a 

 small case only 2 feet in length, 1 foot in height and 1 foot in 

 breadth. The apparatus can be set up in a few minutes, the various 

 adjustments requiring only a short time. 



The radiating apparatus is 6 by 5 by 3 inches, the size of a 

 small lantern. It contains the coil and a small storage cell ; the 

 radiator tube is closed with a thin plate of ebonite to prevent deposit 

 of dust on the radiator. One charge of the cell stores enough 

 energy for experiments to be carried out for nearly a month. It is 

 always ready for use and requires very little attention. A flash of 

 radiation for an experiment is produced by a single tap and break 

 of the interrupting key. 



The radiating apparatus and the receiver are mounted on stands 

 sliding in an optical bench. Experiments are carried out with diver- 

 gent or parallel beams of electric radiation. To obtain a parallel 

 beam, a lens of sulphur or glass is mounted in a tube. Suitable 

 lenses can be constructed from the accurate determination, which 

 I have been able to make, of the indices of refraction of various 

 substances for the electric ray, by a method which will be described 

 later on. This lens-tube fits on the radiator-tube, and is stopped 

 by a guide when the oscillatory spark is at the principal focus of the 

 lens. The radiator-tube is further provided with a series of dia- 

 phragms by which the amount of radiation may be varied. 



For experiments requiring angular measurement, a spectrometer- 



