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A. D. Cole 
converging mirror or lens. It is admissible, however, to use a 
narrow metal strip placed one-quarter wave-length behind it as 
suggested by Righi in his book. We have used a narrow strip of 
sheet metal about 1.5 cm. wide. In fig. 6 abscissas give the angle 
in degrees through which the revolving receiver-arm was rotated 
from its central position and the ordinates the corresponding 
galvanometer deflection. 
(9) A mount of Reflected Energy Depended upon the Angle between 
Plane of Incidence and that of Vibration. By revolving the cylin- 
drical mirrors containing exciter and receiver, the proportion of 
radiation reflected by a surface of liquid is shown to be different 
with exciter and receiver in the relation shown in Fig. 2, from 
that obtained in the position shown in Fig. 2, e. With water and 
an incident angle of 45° these ratios of reflected to incident energy 
are shown to be quite accurately such as are calculated from 
FresneFs well-known formulas for reflection of polarized light, 
taking 8.95 as the refractive index of water for electric waves. 
With alcohol the application of these formulae to the results of a 
reflection experiment show a refractive index smaller than that 
found for longer waves. By this means anomalous dispersion for 
electric waves was discovered,® a discovery independently made by 
Drude a little later by an entirely different method.*^ 
It is believed that the spectrometer form of mounting here 
described considerably facilitates the repetition of such classical 
experiments as are here described, strengthens the force of the 
optical analogies, and provides a suitable means for new research 
work. 
Ohio State University. 
April 28, 1909. 
' H. Hertz, fVied. Ann., 34, p. 610, ’88. 
^ A. D. Cole, Wied. Ann., 57, p. 310, ’96. 
^ P. Drude, tVied. Ann., 58, p. i, 4, 18, ’96. 
