292 Dr. J. H. Vincent on 



This last result is also brought about by under-exposure ; 

 it would be no advantage to increase the brightness of the 

 spark until the exposure was normal. 



Where intensification was employed the mercuric chloride 

 and ammonia method was used. 



Description of Photographs. 



They are all approximately ^ nat. size. 



Fig. 1. Frequency 190. (Frequencies are given only 

 approximately.) 



This illustrates Doppler's Principle. The stand of the fork 

 was placed on a sheet of plate-glass, and was moved by hand 

 across the field of view of the camera. Before the style had 

 reached the portion of the mercury-surface to be illuminated, 

 word was given to a second observer to connect the terminals 

 of the second spark-gap. By the time this could be done the 

 style had entered the field of view and the ripples were 

 thus photographed. The experimental difficulties of pro- 

 ducing such a photograph are similar to the mathematical 

 difficulties met with in investigating the waves given out by 

 a source of finite dimensions moving relatively to the medium*. 

 The moving style, even if not vibrating, would give rise to a 

 series of ripples like those caused by the forward motion 

 of a ship. Thus it was necessary to move the fork slowly 

 enough not to create such disturbances (shown in fig. 2), but 

 yet with sufficient velocity to illustrate Doppler's principle. 



The source is moving in a direction making an angle of 

 about 45° with the edges of the plate. The ripples have a 

 much smaller wave-length in the region which the source is 

 approaching than in that from which it recedes. It should 

 be remarked that in the case of ripples the phenomena are 

 more complicated than in sound or light, where it is usual to 

 consider the effects produced when the velocity of propagation 

 is independent of the wave-length. In the case of ripples the 

 velocity is a function of the wave-length ; the motion of the 

 source relatively to the medium alters the actual velocity in 

 the medium. If it were desired to obtain the exact analogue 

 of the case of sound, or of light in free aether, it would be 

 necessary to use a frequency such that the velocity of the 

 waves was a minimum. The velocity of propagation would 

 then be unaltered by a small change in the wave-length, 



Fig. 2. This is a photograph of the waves caused by the 

 motion of a style which was moved so as to have, as far as 

 possible, only one motion relatively to the mercury surface. 



Fig. 3. Frequencies 190 and 170. 



* Rayleigh's l Sound,' vol. ii. p. 156. 1896 Edition. 



