A Spectrometer for Electromagnetic Radiation 191 
The third object was to secure the means of demonstrating the 
optical analogies of electric waves as completely and rapidly as pos- 
sible in the lecture room. 
It seemed that all three of these objects would be best attained 
by the use of a mounting for the several parts of the wave appara- 
tus similar to a laboratory spectrometer for light. Such a design 
has indeed been used by Righi.“ His apparatus, however, was 
only roughly quantitative and its indications could be seen by but 
a single observer at once. Furthermore, it was clumsy and lacked 
rigidity. 
In the present design, an attempt has been made to secure a 
form sufficiently elastic to adapt it to a wider variety of uses than 
that of either Hertz, Righi or Lodge,® and so develop very com- 
pletely the analogy bet een electrical radiation and light. 
The apparatus consists of a suitable mounting for an exciter or 
generator of electrical waves and a similar mounting for the 
receiver. Each of these is supported by a moveable arm, swing- 
ing horizontally about a common vertical axis which is also the 
axis of a revolving central table. Upon this a prism, grating, 
diffraction slit or other optical device can be placed. The exciter 
and receiver are as a rule each mounted in the focal axis of a 
cylindrical parabolic mirror, as in the original experiments of 
Hertz. Either or both of these converging mirrors however can 
be replaced by a cylindrical lens. An ordinary 5-lb. acid bottle 
I filled with kerosene, benzine or gasoline makes a satisfactory con- 
- centrating lens. If the exciter is placed about 1.5 cm. behind it, 
1 the conditions for a ‘‘parallel beam” are secured. The illustra- 
1 tion, (fig. i) shows the exciter mounted at E behind the lens L and 
the receiver R (enclosed in a small pasteboard box) at the focus 
of the parabolic mirror M. A prism P is placed upon the revolv- 
ing table T so as to receive the radiation concentrated by E and 
refract it to the receiver. The exciter is shown only diagram- 
matically in the figure. It is a modified Righi exciter consisting of 
I two small cylinders with rounded ends, separated by an oil-filled 
^ spark-gap. A continuous flow of kerosene oil passes through this 
I spark-gap. This exciter has been described by the author in 
, Phys. Rev., vol. 23, p. 241, (Sept., ’06). [Some features of it have 
I 
® A. Righi, Die Optik der Electrischen Schwtngungen, p. 9. 
® O. Lodge, The Work of Hertz and Some of His Successors, p. 33. 
