488 Professor A. W. Biicker [March 8, 



If the prong of the tuning-fork moves through the eighty- 

 thousandth of an inch, that is, through a distance equal to a half 

 wave-length of light, the path of the ray which falls upon it is 

 shortened by a whole wave-length, and the position of each band is 

 shifted to that previously held by its neighbour. If the fork vibrates 

 with an amplitude of this almost infinitesimal amount, the bands 

 will disappear, or will alternately appear and disappear according to 

 circumstances. The fork may therefore be used to detect by reson- 

 ance the presence of vibrations, the frequency of which is 64 per second. 

 A priori, there were two difficulties of opposite kinds which made 

 it doubtful whether the fork would be an efficient weapon for the 

 purpose for which it was to be used. 



In the first place it would feel tremors of any sort, and it was 

 doubtful whether it would be possible to discriminate between mere 

 shakes and the vibrations which were to be studied. This difficulty 

 has been very largely overcome. 



The table on which the apparatus stands rests on india-rubber. 

 On the table are a pair of library steps ; these support two pieces of 

 wood, which are heavily weighted and rest on india-rubber balls. 

 From these two beams hang steel wires, which carry india-rubber 

 door-fasteners, and these in turn support two rods on which the 

 paving-stone is placed. By this alternation of elastic and of heavy 

 bodies we can make the bands absolutely steady, unless the disturb- 

 ances are violent. The quiet movements necessary for working the 

 apparatus, the blowing of the bellows, and the like, produce no effect. 

 On the other hand, the shutting of a door in a distant part of the 

 building, the rumble of a cart in the street, will cause the bands to 

 disappear. A great deal of the work on which we rely has been 

 done at South Kensington between midnight and three o'clock in 

 the morning. Trustworthy observations have indeed been made at 

 other times, but it is only in the still small hours that the apparatus 

 is at its best. 



The second doubt was of a different kind. It was certain that the 

 instrument would be more or less shaken ; it was not quite certain 

 whether the fork would respond to vibrations of the given period. 

 It is easy to set a tuning-fork in vibration by resonance when it is 

 mounted on a sounding box, but in that case the vibrations of the 

 enclosed mass of air are communicated through the box to the fork. 

 When the stalk of the fork is held rigidly, a tuning-fork is notoriously 

 difficult to excite by resonance. This objection is, of course, to some 

 extent counterbalanced by the extraordinary sensitiveness of the 

 means of detecting the vibrations, but it is necessary to supplement 

 this by other devices. The instrument used is a siren (S). In front 

 of it is placed a hollow wooden pyramid, the narrow end of which is 

 near to, and is of the same area as the wooden plate attached to the 

 tuning-fork. This serves to collect the waves of sound and to con- 

 centrate them on the fork. Behind the siren is a large resonator by 

 Konig, timed to respond to 64 vibrations per second. 



