[Loupon] A CENTURY OF PROGRESS IN ACOUSTICS 47 
tions, that the particles of the powder necessarily arrange themselves 
in planes at right angles to the direction of the vibratory movements, 
and that their observed distribution at the loops and nodes is in ac- 
cordance with the same laws. 
Method of Slow Movements—Before the invention of the pre- 
ceding methods the acoustician occasionally resorted to the device of 
deducing the vibrations of a sounding body from the behaviour of a 
similar body whose movements were of sufficient amplitude to be seen 
by the eye, and so slow that they could be readily counted. In this 
way Mersenne counted the vibrations of a cord 15 feet long under a 
stretching force of 7 pounds, and found them to be 10 per second. In 
shortening the cord to 1-20 of its length, he obtained an audible sound 
whose pitch he concluded corresponded to 200 vibrations per second. 
In the same way Chladni employed a long and thin metal rod, which 
gave in the first instance only 4 vibrations per second. He then 
shortened the rod until it gave an audible sound, whose pitch he 
determined from the law expressing the relation between the length 
and the number of vibrations. This method, however, which appears 
so simple in theory is subject to large errors and gives in practice very 
poor results. 
Lhe Stroboscopic Method.—Mersenne’s and Chladni’s method 
has accordingly given place to another—the stroboscopic—which allows 
the vibrations of the sounding body to be viewed directly, its move- 
ments relatively to a vibrating eye-piece being rendered as slow as we 
please. The first use of stroboscopic discs for the purpose of observing 
very rapid periodic movements was made by Plateau in 1836. His 
discovery, however, remained unnoticed, for Doppler in 1845 published 
a note on the same subject, without referring to Plateau’s discovery. It 
was Tœpler who first made the method generally known by employing 
it in a series of acoustical experiments, which he published in Poggen- 
Bone MAnnalen” volume 128. In the earlier applications of this 
method, the view of the vibrating body was rendered intermittent by 
looking through slits which were opened and closed in rapid succes- 
sion. This plan was modified by Mach who caused the vibrating body 
tc be illuminated by intermittent light. 
If now we allow the stroboscopic images of a moving body to fall 
on a photographie plate, giving the plate a movement of translation 
which is arrested before each appearance of the image, we thereby 
obtain a series of photographs of the successive positions assumed by 
the body. If, further, matters are so arranged that the beginning and 
duration of the phenomenon are traced on the images, we have a new 
method, which is called Chronophotography. It was M. Janssen who 
first conceived the idea of taking automatically a series of photo- 
