300 Royal Society : — Lord Rayleigh on the Application 



cool the most rapidly, and we shall have, in passing from the out- 

 side to the centre, a series of strata in a more and more perfect 

 crystalline condition. 



Light, as we know in the case of some bodies, tends to promote 

 crystallization, and, when it falls on the surface of such a stick of 

 selenium, probably tends to promote crystallization in the exterior 

 layers, and therefore to produce a now of energy from within out- 

 wards, which under certain circumstances appears, in the case of 

 selenium, to produce an electric current. 



The crystallization produced in selenium by light may also ac- 

 count for the diminution in the resistance of the selenium when a 

 current from a battery is passing through it ; for in changing to 

 the crystalline state selenium becomes a better conductor of elec- 

 tricity. 



" On the Application of the Principle of Reciprocity to Accous- 

 tics." By Lord Rayleigh, E.R.S. 



In a memoir published some years ago by Helmholtz (Crelle, 

 Bd. lvii.) it was proved that if a uniform frictionless gaseous 

 medium be thrown into vibration by a simple source of sound of 

 given period and intensity, the variation of pressure is the same 

 at any point B when the source of sound is at A as it would have 

 been at A had the source of sound been situated at B, and that 

 this law is not interfered with by the presence of any number of 

 fixed solid obstacles on which the sound may impinge. 



A simple source of sound is a point at which the condition of 

 continuity of the fluid is broken by an alternate introduction and 

 abstraction of fluid, given in amount, and periodic according to 

 the harmonic law. 



The reciprocal property is capable of generalization so as to 

 apply to all acoustical systems whatever capable of vibrating about 

 a configuration of equilibrium, as I proved in the Proceedings 

 of the Mathematical Society for June 1873, and is not lost even 

 when the systems are subject to damping, provided that the fric- 

 tional forces vary as the first power of the velocity, as must always 

 be the case when the motion is small enough. Thus Helmholtz's 

 theorem may be extended to the case when the medium is not 

 uniform, and when the obstacles are of such a character that they 

 share the vibration. 



But although the principle of reciprocity appears to be firmly 

 grounded on the theoretical side, instances are not uncommon in 

 which a sound generated in the open air at a point A is heard 

 at a distant point B, when an equal or even more powerful sound 

 at B fails to make itself heard at A ; and some phenomena of 

 this kind are strongly insisted upon by Prof. Henry in opposition 

 to Prof. Tyndall's views as to the importance of " acoustic clouds " 

 in relation to the audibility of fog-signals. These observations 

 were not, indeed, made with the simple sonorous sources of theory ; 

 but there is no reason to suppose that the result would have been 

 different if simple sources could have been used. 



