1894.] 



on the Work of Hertz. 



347 



A block of paraffin about a cubic foot in volume is cast into the 

 shape of a prism with angles 75°, 60°, and 45°. Using the large 

 angle, the rays are refracted into the receiving hat, Fig. 21, and 

 produce an effect much larger than when the prism is removed. 



An ordinary 9-in. glass lens is next placed near the source, and by 

 means of the light of a taper it is focussed between source and re- 

 ceiver. The lens is seen to increase the effect by concentrating the 

 electric radiation. 



The lens helps us to set correctly an 18-in. circular copper disc in 

 position for showing the bright diffraction spot. Eemoving the disc 

 the effect is much the same as when it was present ; in accordance 

 with the theory of Poisson. Add the lens and the effect is greater. 



Fig. 22. 



Zone-plate of Tinfoil on Glass. 

 Every circular strip is of area equal to central space. ' 



With a diffraction grating of copper strips 2 in. broad and 2 in. apart, 

 I have not yet succeeded in getting good results. It is difficult to 

 get sharp nodes and interference effects with these sensitive detectors 

 in a room. I expect to do better when I can try out of doors away 

 from so many reflecting surfaces ; indoors it is like trying delicate 

 optical experiments in a small whitewashed chamber well supplied 

 with looking-glasses; nor have I ever succeeded in getting clear 

 concentration with this zone-plate having Newton's rings fixed to it 

 in tinfoil. But really there is nothing of much interest now in 

 diffraction effects except the demonstration of the waves and the 

 measure of their length. There was immense interest in Hertz's time, 

 because then the wave character of the radiation had to be proved ; 



