December 27, 1901.] 



SCIENCE. 



989 



of any other body may be very accurately 

 compared with the number of alterations 

 marked, in the same time, by the vibrat- 

 ing body." Notwithstanding the clear- 

 ness of this description, the graphical 

 method remained for a long time unknown, 

 and when it was developed later, in 1862, 

 the original discovery was incorrectly at- 

 tributed to Wilhelm Weber (1830). Be- 

 tween these dates slight applications of the 

 method had been made by Savart, Daha- 

 mel, Lissajous and Desains, Wertheim, and 

 others ; the most important of such appli- 

 cations being that of Scott, who in 1858 ap- 

 plied it to his phonautograph. Finally, 

 from 1858 to 1862, Eudolph Koenig devoted 

 himself specially to the perfecting of this 

 method, and exhibited the results of his 

 labors at the Exhibition in London in 1862, 

 in the form of a large collection of phono- 

 grams. This collection in its seven sec- 

 tions comprises all the applications of the 

 method which have so far been made in 

 acoustics. Whilst the progress of this 

 method was thus slow before 1862, its use 

 from that time onward became general, es- 

 pecially in physiological researches, in con- 

 nection with which it received its widest de- 

 velopment in the publication by M. Marey 

 of his splendid work, ' La methode graph- 

 ique' in 1878. Parenthetically I might re- 

 mark that Edison's phonograph (1877) was 

 doubtless suggested by Scott's phonauto- 

 graph. 



As with the graphical method, the earliest 

 suggestion of an optical method of studying 

 vibratory movements camefrom Dr. Thomas 

 Young, who in 1807 gave the construction 

 of curves resultiug from the composition of 

 two rectangular vibratory movements. The 

 practical realization of these curves was 

 effected in 1827 by Wheatstonein his kaleid- 

 ophone. The most important advance, 

 however, in the development of this method 

 was made by Lissajous, who, after some pre- 

 liminary work in 1855, published in 1857 



his great paper entitled ' Memoire sur 

 I'etude optique des movements vibratoires. ' 

 The optical effects produced by Lissajous' 

 method, especially when the curves were 

 projected on the screen, were so beautiful 

 that the method obtained general recogni- 

 tion, and became immediately popularized. 

 The chief merit of the method, however, 

 does not lie in the beauty of the effects thus 

 obtained, but rather in the fact that by this 

 means we are enabled to determine with 

 facility and with the utmost accuracy both 

 the interval and the difference of phase be- 

 tween two vibratory movements. It is this 

 fact which renders the optical comparator 

 one of the most important instruments at 

 the disposal of the acoustician. 



A second optical method we owe to Biot, 

 who in 1820 showed that the changes in 

 density at the nodes of a transparent body 

 vibrating longitudinally could be exhibited 

 when the nodal line of the body is placed 

 between the crossed mirrors of a polariza. 

 tion apparatus. Daring the continuance of 

 the vibrations the image is highly illumin- 

 ated in the analyser and becomes darkened 

 when the vibrations stop. This method 

 was developed much further by Kuudt in 

 1861, and by Mach in 1873. 



A third optical method was devised by 

 Toeplerand Boltzmann in 1870 for the pur- 

 pose of exhibiting the changes which take 

 place at a nodal point of a vibrating column 

 of air. This method consists in producing 

 interference bands by means of two rays of 

 intermittent light from the same source, 

 one of which passes through the air in its 

 normal state, and the other through a nodal 

 point of the vibrating air column. A vibra- 

 tory movement of the interference bands 

 results — a movement which can be made as 

 slow as we please, thus rendering it possible 

 to deduce by stroboscopic methods exact 

 measurements as to the movement of the 

 air at the nodal point. 



The object of the method of manometric 



