ANNIVERSARY ADDRESS. 21 
Though not exactly a matter of the last year or two alone, I 
feel I ought to refer to the work done by Wiener and by 
Lippmann in photographing the effects of standing waves, by 
which I mean the interference effects produced near to a 
reflecting surface by the incident and reflected trains. The 
objects of the two experimenters were by no means identical. 
Wiener deliberately tried to reproduce the experiments of 
Hertz, using light waves and a photographic film, instead of long 
electro-magnetic waves and a resonator. He succeeded in 
making a sensitive collodion film with a thickness of only one- 
thirtieth of the wave length of sodium light—i.e., about one five 
hundred thousandth of a centimeter. By tilting this film with 
respect to the reflecting surface, Wiener was enabled to obtain 
evidence of the nodes and anti-nodes of the chemically active 
Vibrations, in the form of interference strie ; and, moreover, 
was able to show that the strie were standing wave effects, and 
not the ordinary interference effects of thin films. By applying 
' the results of Trouton on the laws of reflection at the polarizing 
angle of electro-magnetic radiation, and assuming that the 
chemical effects are the outcome of the electric rather than the 
Magnetic vibration, Wiener has been able to show that the view 
taken by Fresnel as to the “ direction” of vibration refers to the 
electric vector. It has been suggested that the photographic 
film may be affected by the magnetic component, but most 
physicists would, I think, take Wiener’s view of the matter, as 
being the more probable. 
Lippmann, on the other hand, uses ordinary thick but trans- 
parent films, and obtains many nodes and antinodes in the film 
thickness. In this way he is able to produce a photograph of 
the nature of a diffraction grating, and by a train of reasoning 
which is exceedingly acute, though a good many hypotheses 
are involved, he shows that this grating photograph will yield by 
reflected light a colour which will correspond more or less closely 
with the colour of the light falling on the film originally, and this 
whether the colour was pure or not. Success is the only test of 
