6±8 Mr. G. H. Berry on the 



primary halo and darken its border: of the ray having the 

 definite range of RaA; and, finally, o£ the very penetrating ray 

 giving rise to the corona. It would seem, therefore, that in the 

 pleochroic halo we possess a method of search for new and rare 

 radioactive substances as yet unapproached for sensitiveness.! 



This is due plainly to conditions analogous to those which 

 enable the photographic plate to reveal stars which are too 

 faint to be visually detected, even in the most powerful 

 telescopes. The halo, however, not only integrates the radio- 

 active effects of ages, but presents them to us sorted out 

 according to the laws which govern its formation. 



But there is another aspect of these conditions. The fact 

 that mica, sensitive to the a radiation and capable of inte- 

 grating its effects over geological time, is found unaffected 

 and unaltered in association with many elements in the rocks 

 — elements present in quantities which are enormous relatively 

 to those we have been considering — appears to show con- 

 clusively that these elements do not expel any ionizing 

 u rays even at very prolonged intervals of time. This we 

 appear entitled to accept as testimony of the high stability of 

 many rare and common elements throughout geological time. 



LXVII1. The Striking Point of Pianojorte Strings. 

 By G. H. Beery*. 



[Plates X. & XL] 



IT is the practice of all pianoforte makers to construct 

 their instruments so that the hammers shall strike the 

 strings near the fixed bridge. The well-known explanation 

 of this, first given by Helmholtz, is that striking the string 

 near its end tends to eliminate the upper partials which are 

 dissonant with the fundamental. Experiments carried out 

 by Hipkins t showed that these upper partials, having a node 

 at the point struck, could still be plainly heard, and it would 

 seem that this explanation of Helmholtz is not a sufficient one. 



The object of this paper is to show that an entirely 

 different reason can be given for this preference of striking- 

 place. 



Apparatus. 



Dr. Barton has published % the results of research work 

 on the vibrations of the various parts of the monochord and 



* Communicated by Prof. E. H. Barton, D.Sc, F.R.S.E. 

 ' * ' Sensations of Tone/ translated by Ellis, 3rd ed.ip. 77. 



t Phil, Ma-. July 1905, p. 149: Dec. 1906, p. 576 j April 1907, p. 446 ; 

 Aug. 1909, p. 233. ' 



