hertz's researches on electrical waves. 197 



being cbauged so as to retain complete interference. Or again, 

 whether intersi)ersing throughout the mass of a largo Hertzian pitch- 

 prism, conductors with nearly the same period would alter the angle 

 of refraction. In some such way as this, anomalous dispersion, with 

 its particular case of ordinary dispersion, may yet be successfully 

 imitated. 



The determining the rate of propagation through a large tile, or sheet 

 of sandstone, could be easily made by means of the interference experi- 

 ment, by placing it between the screen and the " resonator." 



EXPERIMENTS ON ELBCTRO-MAGNETIO RADIATION, 



INCLUDING SOME OF THE PHASE OF SECONDARY AVAVES.* 



In continuation of some experiments which were described in Nature, 

 vol. XXXIX, p. 391 (" Repetition of Hertz's Experiments and Determina- 

 tion of the Direction of the Vibration of Light"), attem})ts were made to 

 obtain periodic reflection of electric radiation from plates of different 

 thicknesses, analogous to Newton's rings, with the view of further 

 identifying these radiations with " light." 



It was there described how a sheet of window-glass refused to reflect 

 the Hertzian waves, but how a masonry wall reflected them readily. 

 The non-reflection from the thin sheet is due to the interference of the 

 reflected waves from each side which takes place owing to a change of 

 phase of half a period on reflection at the second surface, as in the black 

 spot of Newton's rings. 



By making the reflection i)late such a thickness that the reflection 

 from the back has to travel half a wave-length farther than that from 

 the front, the two reflections ought to be in accordance, for they differ 

 by a whole period, half arising from difi'erence in path, and half from 

 change of phase on reflection 5 but if the difference in paths were made 

 a whole wave-length by doubling the thickness of the plate, there ought 

 again to be interference, and so on. 



The first plan tried with this end in view, was to fill a large wooden 

 tank to different depths with water or other liquids. On gradually fill- 

 ing the tank reflection should be obtained, and at a certain depth (M[ual 

 to ^ {X sec r) -^ ju, reach a maximum; further addition of the li(piid 

 then should diminish the reflection, and at double the above depth the 

 reflection shoidd reach a minimum, the two waves interfering. 



The mirrors for concentrating the radiation had for this purpose to 



* From Nature, kw^nnt 2^, 1889, vol. xL, pp. 39S-400. 



