158 



Lord Rayleigh on the Passage of 



directions perpendicular to the length of the stalk. In the 

 sketch (tig. 2) AA and BB are circular disks of tin-plate of 



-which A is complete while B is perforated to receive the 

 stalk C. The former is held in position by three distance- 

 pieces extending across, and the diameter A A measures 

 1-J inch (47 mm.). The distance between the plates is £ inch. 

 When the hearing-tube is provided with this appliance as a 

 terminal, sounds are heard with full intensity when they 

 arrive in a direction parallel to the stalk inasmuch as phase- 

 agreement then obtains all round the circumference, but if 

 ■the pitch be that of the whistle a sound arriving in any 

 direction perpendicular to the stalk fails to penetrate C on 

 account of interference. The theory is given in the appendix 

 to this paper. 



I may note in passing that a similar apparatus has been 

 constructed upon a larger scale with disks of thick mill- 

 board 10 inches in diameter and about half an inch apart. 

 This is adapted to pitch c'" of 1024 vibrations per second, 

 and it works well. The note may be given upon a harmonium 

 at a few feet distance. When the stalk points towards the 

 reed the sound is very loud, but it falls off in oblique positions 

 and becomes faint when the reed lies in the plane of the disks. 

 In the open air the apparatus may be used to find the 

 direction in which a sound arrives. 



With these aids I had hoped to be able to eliminate 

 reflexions sufficiently to realize the continuous diminution of 

 intensity which should attend recession from a single source 

 whether situated in the open or on the bounding wall of a 

 semi-infinite space ; but these hopes have been disappointed. 

 In the laboratory an adjustment of the disks to parallelism 

 with the floor (in which the slit was situated) should eliminate 

 reflexions from the walls. There remains the ceiling ; but 

 so far as this was flat it should give rise to alternations with 

 a period of the half wave-length. As a fact the whole wave- 



