ANNIVERSARY ADDRESS. 15 
undoubtedly seemed to be opposed by other results, almost 
equally trustworthy and important. In consequence of this 
discrepancy—for without it the theory must have been only read 
to be accepted—many philosophers waited for “‘ more light ” and 
in the meanwhile worked at matters not immediately depending 
on any theory, such for instance as the exact fixation of the 
electrical units. I will not detain you by referring to the process 
of thought by which Hertz was led to study electrical waves 
produced by purely electrical means, and of a length enormously 
greater than the vaves of light: suffice it to say that he discovered 
a means of producing such waves and was able to show that they 
actually existed and behaved as Maxwell had indicated. At the 
same time, being anxious to establish the main points, Hertz, 
with that rare insight which in experimental work is the chief 
test of genius, laid aside a number of anomalies and unexplained 
results. His followers naturally took up these points and ran 
them down. The first thing was to get some means of detecting 
the nodal points more simple and convenient than the spark gap 
used by Hertz, and accordingly we now have an abundance of 
such means. The latest, the simplest, the most delicate is 
probabably a tube of filings regarded as constituting part of an 
electrical cireuit and whose resistance, viz., that of a large 
number of bad or “ microphone” contacts is found to be affected 
by electrical waves. It was observed by M. Edouard Branly in 
1890* that a bit of glass covered with ‘‘ porphyrised ” copper had 
its resistance enormously diminished if it was placed near to a 
discharging Leyden jar. Avsimilar observation as to the improve- 
ment of a shaking contact under the influence of electrical waves 
had been noticed in 1889 by Prof, Lodge. 
Tf a tube of filings be arranged as one arm of a Wheatstone’s 
bridge, then it is found that the resistance is diminished by the 
impact of even feeble electrical radiation. On tapping or 
shaking the tube, the old resistance approximately reappears. 
Ht has Jong been known that commercial selenium in the 
*C.R. 111, page 785,.and 122, page,90. : 
