46 PRESIDENTS ADDRESS—SECTION A. 
and by constructing a large prism of pitch has found the index 
of refraction of that substance for long waves to which it is of 
course transparent. These measurements are all in as close 
accordance with Maxwell’s theory as could be expected, seeing 
the cittculty there is in making exact measurements of the 
position of so large a body as a resonator. It may be questioned 
whether greater accuracy might not be obtained by the use of 
Geissler tubes, coupled with some system of photography. These 
tubes have already been successfully applied in Dr. Lodge’s 
laboratory, and if it be permissible to prophesy wildly, we may 
see in this observation the germ of a great future development. 
Signalling, for instance, might be accomplished secretly by means 
of a sort of electric ray flasher, the signals being invisible to 
anyone not provided with a properly turned tube. An important 
point in optical theory has been settled lately by the use of 
Hertz’s apparatus in the hands of Fitzgerald and Trouton., 
Assuming the truth of the electro-magnetic “theory for a moment, 
we have to answer the old question as to the relation of the planes 
of the intensities to the plane of polarisation. The answer is 
definite and decisive, and is to the effect that the magnetic inten- 
sity is in the plane of polarisation, and the electric intensity as a 
consequence in the perpendicular plane. Hertz, again, has himself 
constructed a very interesting model of a tourmaline erystal by 
means of wires stretched side by side on a frame. This may be 
considered to form a system in two dimensions, with conductivity 
along one axis, and much less conductivity in the perpendicular 
direction, The behaviour of such a frame to Hertz’s polarised 
rays is exactly equivalent to the action of a plate of tourmaline, 
such as is generally sold for the purpose, on a beam of polarised 
light. This suggests the apparently inevitable conclusion that 
unless energy can be dissipated in some other way than by 
conduction the crystals of tourmaline must have a one-sided con- 
«luctivity. This action must take place in a manner depending on 
the minute structure of the crystal, the variation of conductivities 
along and perpendicular to the-axes of crystals as a whole being 
a well-known and corresponding phenomenon. We turn now to 
some very interesting experiments made by Hertz on the way in 
which the velocity of propagation is influenced by the placing of 
« wire in the field and applying the periodic electric forces to one 
of its ends. For this purpose the flat plate apparatus previously 
-described is furnished with an additional plate placed immediately 
behind, and parallel to one of the flat plates in the original 
apparatus. A wire is led out in front from this plate, and the 
experiment consists in obtaining interference between the radia- 
tion from the wire and the direct radiation from the plates ; as a 
result of these experiments Hertz was led to believe that the 
velocity of propagation is different in a wire from what it is in 
space. Another, and perhaps a better way, is to measure directly 
