174 Dr. J. C. Bose. On the Determination of the 



FIG. 6. 



The Cylindrical Diffraction G-rating. 



gratings were thus made with strips or spaces equal to 3 cm., 

 2*5 cm., 2 cm., 1*5 cm., and 1 cm. respectively. 



The diameter of the cylindrical grating is 100 cm. It would 

 perhaps have been better to use a grating with a less curvature, 

 but it must be remembered that the intensity of radiation is very 

 feeble, and I was apprehensive of the receiver failing to respond 

 when placed at too great a distance. I find from the sensibility of 

 the receiver used that it would be possible to increase the diameter of 

 the cylinder to about 150 cm., and this size I intend to use in the con- 

 struction of my next grating. The aperture of the grating is 

 in the following experiments reduced to the smallest practicable 

 limit. 



Account of the Experiments. 



The receiver being placed at a suitable position on the focal curve, 

 the radiator is moved about on the same curve till the diffracted 

 image falling on the receiver produces response in the galvanometer. 

 The procedure adopted is as follows. The receiver is placed, say, at 

 the centre of the grating (0 = 0). The electric ray at first falls on 

 the grating at a large angle of incidence. A series of flashes of 

 electric radiation are now produced by manipulating the key, and 

 the angle of incidence gradually decreased till the receiver suddenly 

 responds. The angle of incidence corresponding to the zero angle 

 of diffraction is thus determined. The receiver is then placed at a 

 new position on the focal curve, and the corresponding angle of 

 incidence determined as before. In this way a series of angles of 

 incidence, with their corresponding angles of diffraction, are found 

 for each grating. 



