194 
A. D. Cole 
(2) Rectilinear Propagation. (Cf. Wertz, Ausbreitung der elec- 
trischen Kraft, p. 189). For this experiment the two revolving 
arms are brought into the same straight line^ and exciter and 
receiver placed about i meter apart. A metallic screen having 
about the dimensions of the aperture of the parabolic mirrors — 
say 35 cm. square — is placed upon the central table midway 
between tFem. The effect on the leceiver is thus almost entirely 
