Objective Reality of Combination Tones. 



343 



turbances as to be untrustworthy. This difficulty has, how- 

 ever, been entirely overcome. 



A plan of the apparatus is shown in fig. 1. The shank of 

 the tuning-fork, F, is imbedded in a mass of lead. One of 



Fig. 1. 





the prongs carries a mirror, M. To the other is attached a 

 square of wood, "W, of larger area but of the same weight as 

 the mirror. A Lissajous' figure (an ellipse) was formed by 

 reflexion from the mirror and from a small square of silvered 

 glass attached to one of Konig's large forks adjusted to give 

 the C of 64 complete vibrations per second, and the pitch of F 

 was adjusted until only one beat occurred in two minutes. The 

 pitch was thus very accurately known. A double siren, S, 

 was placed between a large Konig resonator tuned to 64 

 vibrations and a wooden cone or pyramid, C. The end of 

 the cone was placed about half an inch from W, which was 

 rather larger than the narrow end of the cone. The sensi- 

 tiveness of the apparatus depended in part upon the distance 

 between W and C. If the distance was too large, the sen- 

 sitiveness was diminished. If it was too small, the instrument 

 was unduly affected by chance puffs of air even when not 

 periodic in character. 



A source of light, L, was used to form a system of inter- 

 ference-bands by means of the half-silvered mirror M 1? and 

 the two mirrors M and M 2 . The two interfering rays 

 travelled over the paths LMiM^B and LMiMMiB respec- 

 tively. The distances MM^ and M^M^ were approximately 

 equal; and since when soda-light was used a change in the 

 length of either of the paths of ^ X 589 fifi would cause a 

 dark band to shift into the position previously occupied by 

 the next bright band, and since, further, any movement of M 

 altered the length of the path of the ray by twice its own 

 magnitude, it is evident that a movement of j X 589 fx/n, or, 

 say, of one hundred thousandth of an inch, could be easily 

 detected. 



The fork F and the mirrors M, M 1? and M 2 rested on a 

 square stone, which was suspended by wires and india-rubber 



2 A2 



