Mr. G. G. Stokes on the Theory of Sound. 203 



plied to the purpose of rendering a meridian mark visible at 

 night; and that the same means might also be used for illu- 

 minating the wires of the transit instrument. I proposed that 

 the meridian mark should consist of a hole in a plate of brass, 

 adjustible in a vertical and horizontal direction by screws, 

 and that behind this hole should be placed the incandescent 

 wire in a glass tube for the purpose of illumination, the bat- 

 tery being of course at any required distance. For illumina- 

 ting the wires of the telescope, I proposed that the platinum 

 wire, protected by a glass tube, should be placed either within 

 or at the side of the eye-piece, and thus obviate the necessity 

 of piercing one of the transit-arms, as is usual. I was only 

 able to tr}' the above plans upon a small scale for want of a 

 more powerful battery ; but the experiments of Mr. Staite 

 lead me to think that there may be cases in which his method 

 of illumination by galvanism might be used with the greatest 

 advantage for rendering a veyy distant meridian mark visible. 

 I have found that even platinum wire, rendered incandescent 

 by alcohol^ may be distinguished by the telescope at a con- 

 siderable distance; as may also the hydrogen and platinum 

 lamp. By any of the above plans, the necessity of attention 

 to the lamp itself by an assistant is done away. 



While upon the subject of micrometer-wires, may 1 also be 

 allowed to state, that so far back as 1831 I invented a substi- 

 tute for them by lines drawn upon glass with a diamond, which 

 lines were illuminated through the edge of the glass? but I 

 was led to abandon the plan after trial, fearing to introduce 

 the errors of the two surfaces of the glass, though I found that 

 in other respects the plan fully answered. I am induced to 

 mention this, having lately seen in the public prints that the 

 same method has been since independently discovered, and I 

 rejoice to find satisfactorily employed, by the Earl Rosse. 



XX VIII. On the Theory of Sound, in reply to Professor 

 Challis. By G. G. Stokes, M.A., Fellow of Pembroke 

 College, Ca m bridge *. 



AS the subject of plane waves does not seem likely to be 

 elucidated by further discussion, I pass on to spherical 

 waves. 



Professor Challis has divided his demonstration of the 

 "contradiction" arrived at in this case into five heads. I 

 entirely agree with the first four ; the fifth I beg leave to dis- 

 pute. The part to which I object is contained in the sentence, 



* Communicated by the Author. iKti^iiiiJi 



