the Photograpluj of Hippies. 295 



A similar effect is shown in this photograph. A strip of 

 aluminium* 2*85 centim.long was cut in a castellated manner 

 so as to have ten bars ; the lengths of the bars and slits were 

 each 1*5 millim. The strip was then bent into a portion of 

 a cylinder of 8 centim. radius. 



The foci are not definite, but the number to be expected 

 agrees with the photograph. The wave-length on the negative 

 is -J 45 centim. The width of a bar and slit is '3 x -755 = '2265 

 centim. The number of diffracted images is thus found by- 

 making sin 6 n equal to unity in 



2*265 sin n =*145tt, 



which shows that we shall only get one diffracted image and 

 thus three foci in all. From the above we find approximately 

 that 



1= 4O°. 

 As near as one can judge this agrees with the angle measured 

 on the negative, taking the centre of the comb as the apex of 

 the angle. 



Fig. 10. Frequency 190. 



This shows the interference pattern produced by three 

 point-sources vibrating in the same phase. 



Fig. 11. The ripples are caused by throwing a leaden shot 

 about 6 millim. diameter into the mercury. This was done 

 by hand. The photograph does not indicate what became of 

 the shot. It probably bounced and was out of the field of 

 view at the instant of illumination. The bright circular 

 patch indicates that the disturbance had subsided in this 

 region. 



Fig. 12. This is a photograph of a ricochet. The dis- 

 turbances were caused by a shot 1*5 millim. in diameter. It 

 first struck the surface inside the larger dark space, then it 

 dropped inside the oval dark space. It probably did not rest 

 here but again left the liquid, and the illumination occurred 

 about this time. If the shot had remained in the second dark 

 space it would have caused a deep depression in the surface, 

 which would have been dark in the print. 



The ripples caused by a splash would form an interesting- 

 research, especially as we already know much about the splash 

 itself from Prof. Worthington's work. 



Description of tome other Experiments. 



In selecting the twelve figures which accompany this paper, 

 regard was paid to the suitability of the negatives for process 

 * Aluminium must be varnished if placed in con tact with mercurv. 



Y2 



