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A. D. Cole 
been described more fully in an earlier paper in the same journal, 
Phys. Rev., voL 7, p. 226 (Nov., ’98)]. The receiver used is a 
Klemencic thermo-junction made of fine iron and Constantin 
wires. An early form is described in Phys. Rev., vol. 4, p. 54 (July 
’96) and the form lately used in Phys'. Rev. ,yo\. 20, ^.26"^ (Apriro5). 
This receiver is used in connection with a low-resistance Kelvin 
galvanometer of fairly high sensitiveness. The receiver is tuned to 
the period of the exciter by use of little sliding tubes as described 
in Phys. Rev., 20, p. 269. Hertz used a receiver whose natural 
period was longer than that of his exciter and Righi one of shorter 
period than that of his exciter, but there are some advantages in 
having both of the same period. 
To avoid confusion in the figure, some details of the apparatus 
are not shown. For instance, each of the parabolic mirrors is 
actually mounted so that it may be revolved about a horizontal 
axis aR. Thus the focal axis of each can be made horizontal or 
vertical, or may be set at any desired angle to that of the other, the 
angular position of each being read on its graduated circle. A 
third graduated arc G shows the angle which the two revolving 
arms, carrying exciter and receiver, make with each other. A 
fourth graduated circle T shows the angle through which the prism 
table is turned. 
To keep the apparatus of convenient dimensions a wave-length 
of 10 to 15 cm. is used. This enables good results to be obtained 
with apparatus of moderate dimensions. For example, the aper- 
ture of the parabolic mirrors is about 35 X 33 cm., the two revol- 
ving arms are one 100 cm. and the other 120 cm., the prism-table 
26 cm. in diameter, prism and lenses 22 cm. high, plane mirrors 
30 cm., square, etc. In contrast with these dimensions Hertz’s 
mirrors were of 200 X 120 cm. aperture, his gratings and his 
smallest plane mirror each 200 X 200 cm.; his prism was 150 cm. 
high and weighed more than 1300 pounds. 
To illustrate the use of the apparatus a brief account follows 
of the method of performing some of the classic experiments of 
Hertz and others who have since brought the optical analogies 
of electrical radiation to convincing completeness, (i) Proof of 
the existence of stationary waves by interference of direct radiation 
with that reflected by a plane surface. The disposition of apparatus 
is shown in Fig. 2 a. The exciter E, mounted in its cylindrical 
mirror, radiates toward the receiver R and the plane mirror M 
