190 The Rev. S. Earushaw on the 



bility, and if the genesis should be so managed as to begin from 

 k=0, and to be capable of changing to fresh values of k, then, 

 from the beginning, sounds corresponding to parts of the curves 

 CB and CD about C would be heard together; but as the 

 genesis went on, the sounds corresponding to the curve C B 

 would get less and less intense, and those corresponding to C D 

 more and more intense ; and the former would gradually become 

 inaudible, and the latter too loud to be borne. Those corre- 

 sponding to A B, if perceived at all, would be most sensible for 

 parts near to B. 



23. Hence the portions both of C B and C D which are appli- 

 cable to the hearing powers of human beings are very limited, 

 and begin from C in both cases. But it has been supposed by 

 Wollaston that the Grylli take up the faculty of hearing where 

 we lay it down ; and there may be animals which take it up 

 where the Grylli lay it down ; and it is thus conceivable that thte 

 whole curve C B is portioned out among different orders of ani- 

 mals from man to the minutest insect. But the curve A B, for 

 the reason pointed out in art. 16, may not belong to waves which 

 affect the organ of hearing at all, but may appeal to some other 

 sense, of which we are probably not possessed, but which is 

 suited to the wants of the minute creatures which the micro- 

 scope has difficulty in revealing. 



The Analogy of Sound and Light. 



24. It is admitted on experimental grounds that a certain 

 degree of analogy exists between sound and light ; and it has 

 been considered allowable to consider the latter as being due to 

 the undulations of an elastic medium consisting of particles 

 separated by finite interval. Hence the preceding investigations 

 and results, after the necessary alteration of numerical constants, 

 will apply to the sethereal medium. There is therefore a maxi- 

 mum limit to the number of free vibrations which the particles 

 of the aethereal medium can execute per second ; and every num- 

 ber from zero to this limit is possible. For this medium also, as 

 lor air, there are two types of wave- disturbance — the circular 

 and the exponential ; and three distinct classes of waves, corre- 

 sponding to three different velocities of transmission for every 

 value of k. There is, in fact, a triplicity of light ; and every 

 degree of velocity from zero to infinity is possible for light as for 

 sound. The figure given in art. 20 preserves the same propor- 

 tions for all media, and will therefore serve for light : the only 

 question that occurs is, what part of it represents sensible light ? 

 Sensible sound, as we have seen, is represented both in the 

 curve C B and in C I) by a small portion of each near to C ; but 

 which parts of the three curves represent sensible effects in the 



