990 



SCIENCE. 



[N. S. Vol. XIV. No. 365, 



flames, invented by Eudolph Koenig in 

 1862, is to furnish an ocular proof of the 

 variations in density at a point of the air 

 traversed by waves originating in another 

 body or in the air itself. A short descrip- 

 tion of the first apparatus based on this 

 method appeared in PoggendoriOf's Annalen 

 in 1864. Between that year and 1872 the 

 method was applied to a series of instru- 

 ments, the experiments being described in 

 the same journal in a long memoir entitled, 

 ' Les flammes manometriques.' Although 

 this method is extremely sensitive and 

 capable of furnishing very accurate results, 

 it has been prevented for a long time from 

 rendering more efficient service on account 

 of two causes : first, the want of sufficient 

 brightness in the reflected images of the 

 jumping flames, and second, the difficulty 

 of observing the details of these images 

 owing to their momentary appearance in 

 the mirror. The former of these difficulties 

 has now been overcome by the employment 

 of acetylene and other gases, which at the 

 same time allow admirable photographs of 

 the flames to be taken, thus obviating the 

 second difficulty also. We owe an impor- 

 tant paper on this subject to Professors E. L. 

 Nichols and Ernest Merritt published in 

 1898 in the Physical Review. 



In 1865, Kundt published his method of 

 using light powders for the purpose of ex- 

 hibiting the vibratory character of station- 

 ary. air waves in columns and plates of air. 

 During the existence of these vibrations the 

 light powders arrange themselves in trans- 

 versal strise which collect around the loops, 

 and are wanting at the nodes. As in the 

 case of the nodal lines on Chladni's plates, a 

 satisfactory explanation of these strise was 

 for a long time wanting. In 1890 Profess- 

 or Walter Koenig showed, from hydrody- 

 namical considerations, 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 accordance with the same laws. 



Before the invention of the preceding 

 methods the acoustician occasionally re- 

 sorted to the device of deducing the vibra- 

 tions of a sounding body from the behavior 

 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 ^V 

 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. 



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 movements 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 un- 

 noticed, for Doppler, in 1845, published a 

 note on the same subject, without referring 

 to Plateau's discovery. It was Toepler 

 who first made the method generally known 

 by employing it in a series of acoustical ex- 

 periments which he published in Poggen- 

 dorff's Annalen, 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 



