[Loupon] A CENTURY OF PROGRESS IN ACOUSTICS 45 
may be very accurately compared with the number of alternations 
marked, in the same time, by the vibrating body”. Notwithstanding 
the clearness of. this description, the graphical method remained for a 
long time unknown, and when it was developed later in 1864 the ori- 
ginal discovery was incorrectly attributed to Wilhelm ,Weber (1830). 
Between these dates slight applications of the method had been made 
by Savart, Duhamel, Lissajous and Desains, Wertheim and others; the 
most important of such applications being that of Scott, who in 1858 
applied it to his Phonautograph. Finally, from 1858 to 1862, Rudolph 
Koenig devoted himself specially to the perfection of this method, and 
exhibited the results of his labours at the Exhibition in London in 
1862, in the form of a large collection of phonograms. This collec- 
tion in its seven sections 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, especially in physiological researches, in connection 
with which it received its widest development in the publication by 
M. Marey of his splendid work, “La Méthode graphique” in 1878. 
Parenthetically I might remark that Edison’s Phonograph (1877) was 
doubtless suggested by Scott’s Phonautograph. 
Optical Methods——As with the graphical methods, the earliest 
suggestion of an optical method of studying vibratory movements came 
from Dr. Thomas Young, who in 1807 described the construction of 
curves resulting from the composition of two rectangular vibratory 
movements. The practical realization of these curves was effected in 
1827 by Wheatstone in his Kaleidophone. The most important ad- 
vance, however, in the development of this method was made by 
Lissajous who, after some preliminary work in 1855, published in 
1857 his great paper entitled “ Mémoire sur l’étude optique des mouve- 
ments 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 recognition, and be- 
came immediately popularized. The chief merit of the method, how- 
ever, 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 facil- 
ity and with the utmost accuracy both the interval and the difference 
of phase between 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 polarization apparatus. During 
