ACOUSTICS AND GRAVITATION. 



61 



in the wave band from oscillations of the plate around a horizontal axis are 

 often striking in the former case. Thus a succession of strong waves subsiding 

 to fringe bands passing through a gap and then in turn to a line and a strong 

 wave, are frequent in intense vibrations. The gaps are often in multiple with 

 pulses between, as in figure 87. Waves seem to tear themselves apart longi- 

 tudinally, leaving a gap between, which in turn closes by reversed motion. 



There is a correlative cause for the possible occurrence of gaps which must 

 not be lost sight of. Since the fringe vibration up and down is very fast in 

 frequency compared with the vibration of the objective, it will be chiefly at 

 the maxima and minima, where the up-and-down motion is instantaneously 

 zero, that they will be most easily seen in the vibration telescope, even 

 when the mirror of the telephone is displaced translationally. Unless the (to- 

 and-fro) amplitude of the objective is very long, the fringes will overlap in 

 their up-and-down motion, leaving gaps in these places in the field. It is 

 these beaded forms and gaps which which may often be transformed into 

 waves, by making the objective travel more swiftly, i. e., to fall, from a 

 larger amplitude. When, however, gaps occur in alternation with waves, as 

 suggested in figure 87, it does not seem probable that they can be produced 

 otherwise than by plate rotation, which slides the slit-images longitudinally 

 along each other till the fringes actually vanish. 



54. Synchronism. Iron-screw core. To obtain further insight into these 

 phenomena, it is necessary to make the period of the vibrating objective 

 and of the interrupter of the telephone current identical 

 and incidentally to devise means for increasing the 

 amplitude of the objective as far as possible. For 

 this purpose the apparatus, figure 88, was designed. 

 Here M is the usual electromagnet held in a sleeve H, 

 with a stem a, fitting the clutch on a vertical standard 

 (not shown), so that M may be raised or lowered, placed 

 nearer to or further from the steel spring s. The latter 

 is shaped like a hacksaw blade, firmly clamped in a small 

 vise v below. This, also, is capable of being raised or low- 

 ered on the specified standard. The spring 5 carries the 

 light lens L, which is the objective of the telescope. L 

 need not be achromatic (in which case it would be too 

 heavy), for the fringes appear almost equally well with 

 a simple lens, if the focal length is suitable to the telescope. 



The standard rod to which the vise v and the electro- 

 magnet M are adjustably clamped must be rigidly attached to the tube of 

 the telescope, which has the usual biaxial mounting on a tripod. Hence the 

 telescope may be pointed in any direction while the lens L is in uninterrupted 

 vibration. This is a great convenience in viewing different parts of the 

 fringe band, drawn out by the quivering objective. 



