80 Intelligence and Miscellaneous Articles. 



f o m cos 2 to -j- f 91 



—± — = cos~a, t— ^=cos / a. 



m+f m+f 



Let it be remarked that we may suppose m=I, and that the two 

 terms of the second ratio should be multiplied by the same factor 

 variable with w, since the light emitted varies with the depth of the 

 luminous thread, and this changes with the inclination. Elimina- 

 ting/in these two equalities, we get 



sin a' = sin a sin u>. 

 I have verified this relation with a thin-walled spherical balloon 

 filled successively with very pure alcohol and hydride of hexyl. 

 Without entering into the detail of the experiments and the pre- 

 cautions taken to realize the equality of the tints of the two images 

 (a condition without which the equality of the lights becomes illu- 

 sory), I may say that the law was very well verified on causing tv to 

 vary from zero to 65° ; the errors in the determinations of a never 

 exceeded 1°, which exhibits a very sufficient approximation for pho- 

 tometric measures. — Comptes Rendus de VAcademie des Sciences, vol. 

 lxxvii. pp. 1216-1219. 



ON A PROCESS FOR VERIFYING THE NODES IN A SOUNDING PIPE. 

 BY M. BOURBOUZE. 



The nodes of vibration in pipes are the places where the air is 

 motionless, but where it undergoes alternate compressions and di- 

 latations synchronous with the duration of the vibration. They 

 are usually verified by showing that a membrane covered with sand, 

 introduced into the tube, does not vibrate. 



M. Koenig has contrived placing in the side of the pipe a capsule 

 closed interiorly by a flexible membrane, and through which circu- 

 lates a current of illuminating gas, which is lighted. When the 

 pipe carries the compressed membrane, it alternately dilates the 

 current of carburetted hydrogen, and the flame undergoes oscilla- 

 tions which are ascertained by viewing them in a revolving mirror. 

 This procedure is excellent, but does not lend itself to the projec- 

 tions which are necessary in lectures. 



I replace these capsules by a simple membrane of flexible caout- 

 chouc, upon which I fix a very light silvered mirror, so that it 

 oscillates with the membrane. Consequently, if the rays from a 

 luminous point be thrown upon this mirror, and the image be pro- 

 jected by a lens, the image will be seen to elongate, as in the expe- 

 riments of M. Lissajous, and frequently to be transformed into an 

 ellipse. It has its maximum of elongation when the mirror is at 

 the node ; it approaches immobility, and at length remains motion- 

 less, as the mirror is removed from the node to be placed on a. loop. 



This membrane can be placed at the extremity of a Helmholtz 

 resonator, or at the end of a caoutchouc tube fixed to the extremity 

 of that instrument ; and we can assure ourselves that the mirror 

 vibrates when we produce in the vicinity a mixed sound containing 

 the note proper to the resonator. 



The new process replaces with advantage, in lectures and in re- 

 rearches of investigation, those which have till now been made use 

 of. — Cornptes Rendus de VAcademie des Sciences, vol. lxxvii. p. 1099. 



