lioval Snstitutxon of (Br^at 



WEEKLY EVENING MEETING 

 Friday, January 15, 1904. 



Sir Benjamin Baker, K.C.B. K.C.M.G. LL.D. F.R.S. M.Inst.CE., 



Vice-President, in the Chair. 



The Right Hon. Lord Eayleigh, O.M. M.A. D.C.L. LL.D. Sc.D. 

 F.R.S. M.B.L, Professor of Natural Philosophy, R.I. 



Shadows. 



My subject is shadows, in the literal sense of the word — shadows 

 thrown by light, and shadows thrown by sound. The ordinary 

 shadow thrown by light is familiar to all. When a fairly large 

 obstacle is placed between a small source of light and a white screen, 

 a well-defined shadow of the obstacle is thrown on the screen. This 

 is a simple consequence of the approximately rectilinear path of 

 light. Optical shadows may be thrown over great distances, if the 

 light is of sufficient intensity : in a lunar eclipse the shadow of 

 the earth is thrown on the moon : in a solar eclipse the shadow of the 

 moon is thrown on the earth. Acoustic shadows, or shadows thrown 

 by sound, are not so familiar to most people ; they are less perfect 

 than optical shadows, although their imperfections are usually over- 

 estimated in ordinary observations. The ear is able to adjust its 

 sensitiveness over a wide range, so that, unless an acoustic shadow 

 is very complete, it often escapes detection by the unaided ear, the 

 sound being sufficiently well heard in all positions. In certain 

 circumstances, however, acoustic shadows may be very pronounced, 

 and capable of easy observation. 



The difference between acoustic and optical shadows was considered 

 of so much importance by Newton, that it prevented him from accept- 

 ing the wave theory of light. How, he argued, can light and sound 

 be essentially similar in their physical characteristics, when light 

 casts definite shadows, while sound shadows are imperfect or non- " 

 existent ? This difficulty disappears when due weight is given to the 

 consideration that the lengths of light waves and sound waves are of 

 different orders of magnitude. Visible light consists of waves of 

 which the average length is about one forty-thousandth of an inch. 

 Audible sound consists of waves ranging in length from about an inch 

 to nearly forty feet : the wave length corresponding to the middle C of 

 the musical scale is roughly equal to four feet. It is, therefore, no 

 matter for wonder that the effects produced by sound waves and by 

 light waves differ in important particulars. 



Moreover, the wave length is not the only magnitude on which the 

 perfection of the shadow depends ; the size of the obstacle, and the 



Vol. XVII. (No. 98.) 2 g 



