46 ROYAL SOCIETY OF CANADA 
the continuance of the vibrations the image is highly illuminated in 
the analyser and becomes darkened when the vibrations cease. This 
method was developed much further by Kundt in 1864 and by Mach 
in 1873. 
A third optical method was devised by Tœpler and Boltzmann in 
1870 for the purpose 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 vibratory 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. 
Method of Manometric Flames.—The object of the method of 
manometric flames, invented by Rudolph Keenig in 1862, is to furnish 
an ocular proof of the variations in density at a point of the air tra- 
versed by waves originating in another body or in the air itself. A 
short description of the first apparatus based on this method appeared 
in Poggendorfi’s “Annalen” in 1864. Between that year and 1872 the 
method was applied to a series of instruments, the experiments being 
described in the same Journal in a long memoir entitled “Les Flammes 
manométriques”. 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 de- 
tails of these images owing to their momentary: appearance in the q 
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 important paper on this subject — 
to Professors E. L. Nichols and Ernest Merritt, published in 1898 m 
the “ Physical Review ”. q 
Kundt’s Method—In 1865 Kundt published his method of © 
using light powders for the purpose of exhibiting the vibratory char- | 
acter of stationary air waves in columns and plates of air. During the | 
existence of these vibrations the light powders arrange themselves im 
transversal striæ which collect around the loops, and are wanting at M 
the nodes. As in the case of the nodal lines on Chladni’s plates, 2, 
satisfactory explanation of these strie was for a long time wanting. In 
1890 Professor Walter Koenig showed from hydrodynamical considera- # 

