Prof. Tyndall on the Transmission of Sound. 151 



by Helrnholtz, viz. that the low-tint colours (couleurs degradees), 

 such as russet, brown, olive-green, peacock-blue, &c, are the result 

 of relatively low illumination. He mentioned that he obtained 

 these effects by diminishing the intensity of the light in the colours 

 to be examined, and by, at the same time, maintaining a brilliantly 

 illuminated patch in an adjoining part of the field of view. If 

 therefore we use the combination JN", Q, P, P x (i. e. if we remove 

 the second quartz plate), we can, by turning the prism P round, 

 diminish to any required extent the intensity of the light in one 

 pair of the complementary images, and at the same time increase 

 that in the other pair. This is equivalent to the conditions of 

 Helmholtz's experiments ; and the tints in question will be found 

 to be produced. 



" Further Experiments on the Transmission of Sound." By 

 John Tyndall, D.C.L., LL.D., E.K.S., Ac. 



The author describes a number of experiments made with hete- 

 rogeneous atmospheres obtained by saturating alternate layers of air 

 with the vapours of various volatile liquids. Starting from his ob- 

 servation on the transmission of sound through a snow-storm on 

 the Mer de G-lace, in the winter of 1859, he shows the extraordi- 

 nary power of sound to pass through the interstices of solid bodies 

 as long as the continuity of the air is preserved. Sound, for example, 

 penetrates through twelve layers of a silk handkerchief, while a 

 single layer of the same handkerchief dipped into water, so as to 

 fill the interstices, cuts off the sound. 



He also describes numerous experiments with artificial fogs 

 of a density so great that a depth of three feet sufficed to inter- 

 cept the concentrated beam of the electric light ; the effect of 

 such fogs on sound was sensibly nil. Experiments were also exe- 

 cuted on the illumination of such fogs by sudden flashes, obtained 

 by the combustion of gunpowder or gun-cotton, or by the alternate 

 extinction and revival of the electric and other lights. Such flashes 

 promise to be extremely useful as fog-signals. 



The author corrects the mistake of supposing that, in the expe- 

 riments at the South Eoreland, the lower trumpets were not com- 

 pared with the higher ones. This, in fact, was the first step of 

 the inquiry. 



He also communicated an extraordinary instance of the intercep- 

 tion of sound during one of the battles of the late American war. 



In these experiments the'author has been ably aided by his assist- 

 ant, Mr. John Cottrell. An account of the experiments will be 

 found in a paper now printing for the Philosophical Transactions. 



