6 Light and Sound. [Jan., 



curious and well-established instance (though probably by no means 

 the only one), the analogy runs very close. It has been observed 

 that in wood sound is conducted with different facility in three 

 directions. Lengthways, or along the grain, the conduction is best ; 

 across the rings or grain it is much worse ; and tangential to the 

 rings it is worst of all. Exactly the same facts, and in the same 

 order, have been found to hold good as regards the conduction of 

 heat in wood. But what is true of the conduction of heat is in 

 every case, even in this last, equally true of the conduction of 

 another force, namely, electricity. Now the conduction of electricity 

 strongly resembles the conduction of sound ; inasmuch that while the 

 rate of propagation of so und through liquids is increased by increase 

 of temperature, it is decreased in solids by the same cause. So like- 

 wise it is found that the same causes produce the same effects on 

 similar substances in the rate of propagation of electricity. Thus, 

 in the first place, we link light on to heat, and then heat on to 

 sound ; after that, heat on to electricity, and here, at last, electricity 

 on to sound, whilst the connection between sound and light it is our 

 object to set forth in this article. How this incidental fact opens up 

 for a moment the oneness of the diverse forces which play around the 

 world ! We are encircled with wonder and with mystery, but every 

 now and then facts such as these arise, which lead us to believe that 

 perfect unity and simplicity he somewhere in the back-ground. 



§ 3. The Peeception of Light and Sound. 



At first it would appear hopeless to seek for any analogy 

 between organs so essentially different as the eye and ear. It 

 must, however, be borne in mind that the functions of those organs 

 are not only to receive the impressions of light and sound, but also 

 to gather up and suitably present the wave-motion which impinges 

 upon them. Owing to the vast difference in tenuity and elasticity 

 of the media which convey light and sound, we should expect to 

 find the very difference we observe in the apparatus contrived for 

 hearing and seeing. Nevertheless it is possible to trace some cor- 

 respondence of parts in the eye and the ear ; and there have not 

 been wanting physiologists who have pushed this view to a detailed 

 and fanciful extreme. The reader who wishes for further informa- 

 tion on this presumed resemblance of the two organs will find it 

 given in a recent work by Dr. Macdonald,* to which we may have 

 occasion again to refer. 



But there is a remarkable analogy, very much overlooked, 

 which appears to hold good between the perception of the respec- 

 tive impressions of light and sound. When we consider the 

 multitude and complexity of the sounds we hear in a concert, and 

 when we remember the extent, the diversity of colour and appear- 



* ' Sound and Colour,' by Dr. Macdonald, F.R.S. Longmans, 1869. 



