1887.] Lord Bayleigh on Diffraction of Sound. 187 



WEEKLY EVENING MEETING, 



Friday, January 20, 1888. 



William Huggins. Esq. D.C.L. LL.D. F.R.S. Vice-President, 

 in the Chair. 



The Eight Hon. Lord Katleigh, M.A. D.C.L. LL.D. F.R.S. M.B.L 



PROFESSOR OF NATURAL PHILOSOPHT, K.I. 



Diffraction of Sound. 



The interest of the subject which I propose to bring before you this 

 evening turns principally upon the connection or analogy between 

 light and sound. It has been known for a very long time that sound 

 is a vibration ; and every one here knows that light is a vibration 

 also. The last piece of knowledge, however, was not arrived at so 

 easily as the first; and one of the difficulties which retarded the 

 acceptance of the view that light is a vibration was that in some 

 respects the analogy between light and sound seemed to be less 

 perfect than it should be. At the present time many of the students 

 at our schools and universities can tell glibly all about it ; yet this 

 difficulty is one not to be despised, for it exercised a determining 

 influence over the great mind of Newton. Newton, it would seem, 

 definitely rejected the wave theory of light on the ground that 

 according to such a theory light would turn round the corners of 

 obstacles, and so abolish shadows, in the way that sound is generally 

 supposed to do. The fact that this difficulty seemed to Newton to be 

 insuperable is, from the point of view of the advancement of science, 

 very encouraging. The difficulty which stopped Newton two cen- 

 turies ago is no difficulty now. It is well known that the question 

 depends upon the relative wave-lengths in the two cases. Light- 

 shadows are sharp under ordinary circumstances, because the wave- 

 length of light is so small : sound-shadows are usually of a diffused 

 character, because the wave-length of sound is so great. The gap 

 between the two is enormous. I need hardly remind you that the 

 wave-length of C in the middle of the musical scale is about 4 feet. 

 The wave-length of the light with which we are usually concerned, 

 the light towards the middle of the spectrum, is about the forty- 

 thousandth of an inch. The result is that an obstacle which is 

 immensely large for light may be very small for sound, and will 

 therefore behave in a different manner. 



That light-shadows are sharp is a familiar fact, but as I can prove 

 it in a moment I will do so. We have here light from the electric 

 arc thrown on the screen ; and if I hold up my hand thus we have a 

 sharp shadow at any moderate distance, which shadow can be made 



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