A Spectrometer for Electromagnetic Radiation 195 
I cut ofFj but only when the screen center is on the straight line 
connecting exciter and receiver. Ordinary sheet zinc is used 
I for the screen. 
I (3) Polarization, In place of Hertzes huge polarization grating 
of parallel wires, we use a piece of cardboard, 35 cm. square, pro- 
vided with parallel strips of tinfoil, each 2 mm. wide and i cm. 
apart. 
(4) Reflection. Hertzes experiment showing the equality of the 
angles of incidence and reflection by a plane mirror, is readily 
repeated with the 35 cm. square sheet of zinc used in 2. The 
arrangement of apparatus is shown diagrammaticaliy in fig. 2, h. 
(5) Refraction. This is illustrated already in the action of the 
'Tottle lens.’’ Its efficiency in concentrating the radiation is 
easily shown by removing it in such an experiment as the last 
named. The eff*ect on the receiver immediately falls to about one- 
fifth that before obtained. The experiment which Hertz performed 
with his gigantic ijoo-lb. prism of street asphalt we have repeated, 
first with a hollow prism made of window glass having faces about 
.15 X 20 cm. and refracting angle 30°, filled with resin-oil. When 
water or alcohol is placed in the hollow prism, no measurable 
fraction passes through. Another larger prism of solid resin, with 
30° angle and faces 25 cm. square, was later constructed and gave 
good results; showing a deviation of about 18°. This prism has 
become broken, and one of a less brittle material, hard paraffine, is 
being made to take its place. 
(6) Interference of Two Reflected Waves. This famous experi- 
ment of Boltzmann is readily performed. The necessary arrange- 
, ment of apparatus is shown in fig. 2, c. In this case two mirrors 
j M and are used. At first both are in the same plane and act 
i as one large mirror, reflecting the radiation received from E to the 
I receiver at R. Then is shifted back a few centimeters at a 
I time and readings taken for each position. A figure showing the 
j interference curve for this case was shown in the paper in Phys. 
Rev., vol. 20, p. 271, and is reproduced here as fig. 4. 
(7) Refractive Index hy Interposed-plate Method. By inserting a 
plate of dielectric material in the path of one of the interfering 
I beams in the last experiment, the position of nodes and loops will 
be shifted because the radiation moves with diminished velocity 
in the dielectric medium. From the amount of this displacement 
the refractive index is readily calculated. We have used paraffine 
