456 Lord Rayleigh's Acoustical Observations. 



Observed. Calculated. 



nd> 11 2 10 59 



n,4> 33 331 33 19 



</></>, 22 331 22 31 



c<j> 27 42i 27 47J 



Gold. 

 Measurement of a la roe though imperfect crystal in the 

 British Museum showed it to be a combination of the cube 

 with the tetrakishoxahedron {4 1 0} and the triakisoctahedron 

 {811}. As the faces were very dull, and but rough measure- 

 ments could be obtained, I was glad to confirm this observa- 

 tion by the examination of a crystal showing the same com- 

 bination in Mr. Ludlam's beautiful collection, which he was 

 good enough to lend me. The angles measured on the latter 

 crystal agree fairly well with the calculated angles. 



Measured. Calculated. 







(410,811) 9 25 9 52 



(410,401) 20 18 19 44 



I saw recently a very beautiful crystal of tbe same combi- 

 nation in the collection of the Ecolo des Mines, Paris. In the 

 two former the planes {811} are deeply striated parallel to 

 their intersection with the faces of the cube. 



LXI. Acoustical Observations. 

 By Loud Bayleigh, M.A., F.R.S.* 



Perception of the Direction of a Source of Sound. 



IN a paper with the above title, communicated last year to 

 the Musical Association and afterwards published in abs- 

 tract in ' Nature,' I brought forward the fact that we are 

 unable to distinguish whether a pure tone (obtained from a 

 tuning-fork and air-resonator) is immediately in front of or 

 immediately behind us — although with other sounds, and no- 

 tably with the human voice, there is in general no difficulty. 

 In order to make the experiment satisfactorily, it is necessary 

 to provide two similar forks and resonators and to place the 

 observer between them. At a given signal both forks are 

 struck, but one of them only is held over its resonator. If 

 this precaution be neglected, the noise attending the oxcita- 

 tion of the fork vitiates the experiment. Subsequently to the 

 reading of my paper, it occurred to mo that if the ordinary 



* Communicated by the Author. 



