416 Mr. J. H. Vincent on the 



sources. These fringes are not seen on the screen in 

 M. Meslin's experiment with the split lens, even when the 

 screen is placed beyond the second focus, because the pencils 

 do not there overlap. It seems that modifications of 

 M. Meslin's experiment could be devised so as to enable 

 complete circular fringes to be seen, and also to render the 

 sections of hyperboloids visible. For example, it appears 

 probable that if a circular portion of a convex lens were cut 

 out and the central portion moved towards the original point- 

 source, the sections of hyperboloids of revolution would be 

 visible on a screen placed beyond the second focus. 



Fig. 4. The two sets of ripples are produced by a fork 

 of frequency 128 and another of frequency about 112. 

 These two forks then produce 16 beats a second. The curved 

 light lines represent places of minimum disturbance at the 

 instant when the spark occurred. These lines are not 

 stationary as in No. 3, but rotate towards their convexities. 

 ' The centre of disturbance from which they move is the one 

 of higher frequency. If we consider a point anywhere on 

 the surface of the mercury, beats occur at that point with the 

 same frequency as the passage of these lines of minimum 

 disturbance takes place over the point. Thus, 16 of these 

 lines cross any point per second. 



Fig. 5. This shows ripples produced by two forks, the 



hioher of which has a frequency four times as great as the 



lower, the frequency of Avhich is 128. If we neglect the 



effect of gravity, 2 ttT 



v 2 = n z \" = -, 



Xp ■ 



from which it follows that the wave-length of the ripples 

 from the higher fork should be half that due to the lower. 

 This relation is approximately true for these ripples. 



Fig. 6. Frequency 180. 



This photograph shows a point-source and a reflecting-line, 

 the latter is a side of a triangular piece of microscope cover- 

 glass, which is kept in position by a small splinter of wood. 

 The interference-lines which are shown are due to the mutual 

 action of the primary and the reflected waves. The phenomena 

 exhibited are analogous to Lloyd's single-mirror fringes in 

 Optics. 



Faint signs of diffraction invading the geometrical shadow 

 of the obstacle can be seen. The region of shadow is covered 

 by faint lines parallel to the nearest side of the triangle acting 

 as a line-source. The wave-length is the same as that of the 

 primary waves, and the effect is due to forced vibrations. 



Fig. 7. This photograph illustrates reflexion and forced 



