Surfaces, Clean and Contaminated, 389 



an intermittent one, isoperiodic with the vibrations themselves, 

 and may be obtained in the manner first described by Plateau. 

 In the present case it was found simplest to render the light 

 itself intermittent. Close in front of the small gas-flame was 

 placed a vibrating blade of tin-plate rigidly attached to the 

 extremity of the prong of a large tuning-fork, and so situated 

 that once during each vibration the light was intercepted by 

 the interposition of the blade. The vibrations of the fork 

 were maintained electromagnetically in the usual manner, and 

 the intermittent current furnished by the interrupter fork 

 was utilized, as in HelmhohVs vowel-sound experiments, to 

 excite a second, in unison with itself. The second fork gene- 

 rated waves in the dish of water by means of a dipper attached 

 to its lower prong. 



When the action is regular, the vibrations of the two forks 

 are strictly isochronous, even though the natural periods may 

 differ somewhat*. The view presented to the observer is then 

 perfectly steady, and corresponds to one particular phase of 

 the vibration, or rather, since the illumination is not instan- 

 taneous, to an average of phases in the neighbourhood of a 

 particular one. 



Even in the case of a perfectly regular train of waves, the 

 appearance will depend upon the precise position occupied by 

 the eye. It is evident that the light most diverted from its 

 course is that reflected from the shoulders of the waves — the 

 points midway between the troughs and crests, for it is here 

 that the slope of the surface is greatest. Thus if the eye be 

 moved laterally outwards from the focal point, until all light 

 has nearly disappeared, the residual illumination will mark 

 out the instantaneous positions of one set of shoulders, all other 

 parts of the complete wave remaining dark. This is one of 

 the most favourable positions for observation. If the devia- 

 tion from the focal point be in the opposite direction, the 

 other set of shoulders will be seen bright. 



The aspect of the waves was not always equally pleasing. 

 Sometimes the formation of stationary waves, due to reflexions, 

 interfered with regularity. A readjustment of the walls of 

 the vessel relatively to the dipper would then often effect an 

 improvement. The essential thing is that there should be no 



* A dirty condition of the mercury sometimes leads to the failure of 

 several successive contacts. During the interval the vibrations of the 

 second fork, being unconstrained, take place in their natural period. In 

 this way a phase-discrepancy may set in, to be subsequently corrected 

 when the regular contacts are reestablished. Such a state of things is to 

 be avoided as distracting to the eye, and unfavourable to accurate obser- 

 vation. 



