TRANSACTIONS OF SECTION A. 703° 
difficulties in its way seem to me considerably less than that of supposing that\a 
wave in the ether can change its velocity when moving from point to point in a 
uniform magnetic field. tts 
I now pass on to the consideration of the Rontgen rays. We are not yet 
acquainted with any crucial experiment which shows unmistakably that these rays 
are waves of transverse vibration in the ether, or that they are waves of normal 
vibration, or indeed that they are vibrations at all. Asa working hypothesis, how- 
ever, it may be worth while considering the question whether there is any property 
known to be possessed by these rays which is not possessed by some form or other 
of light. The many forms of light have in the last few months received a note- 
worthy addition by the discovery of M. Becquerel of an invisible radiation, possess- 
ing many of the properties of the Réntgen rays, which is emitted by many fluores- 
cent substances, and to an especially marked extent by the uranium salts. By 
means of this radiation, which, since it can be polarised, is unquestionably light, 
photographs through opaque substances similar to, though not so beautiful as, those 
obtained by means of Réntgen rays can be taken, and, like the Rontgen rays, they 
cause an electrified body on which they shine to lose its charge, whether this be 
positive or negative. 
- The two respects in which the Réntgen rays differ from light is in the 
absence of refraction and perhaps of polarisation. Let us consider the absence 
of refraction first. "We know cases in which special rays of the spectrum pass: 
from one substance to another without refraction; for example, Kundt showed 
that'gold, silver, copper, allow some rays to pass through them without bending, 
while other rays are bent in the wrong direction. Pfliiger has lately found that» 
the same is true for some of the aniline dyes when in a solid form. In addition to 
this, the theory of dispersion of light shows that there will be no bending when the 
frequency of the vibration is very great. 1 have here a curve, taken from a paper 
by Helmholtz, which shows the relation between the refractive index and the 
frequency of vibration for a substance whose molecules have a natural period of 
vibration, and one only ; the frequency of this vibration is represented by OK in the 
diagram. ‘The refractive index increases with the frequency of the light until the 
latter is equal to the frequency of the natural vibration of the substance; the 
refractive index then diminishes, becomes less than unity, and finally approaches 
unity, and is practically equal to it when the frequency of the light greatly exceeds 
that of the natural vibration of the molecule. Helmholtz’s results are obtained on 
the supposition that a molecule of the refracting substance consists of a pair of 
oppositely electrified atoms, and that the specific inductive capacity of the medium. 
consists of two parts, one due to the ether, the other to the setting of the molecules 
along the lines of electric force. 
Starting from this supposition we can easily see without mathematical analysis: 
that the relation between the refractive index and the frequency must be of the 
kind indicated by the curve. Let us suppose that an electromotive force of given 
amplitude acts on this mixture of molecules and ether, and let us start with the fre- 
quency of the external electromotive force less than that of the free vibrations of the: 
molecules : as the period of the force approaches that of the molecules, the effect of 
the force in pulling the molecules into line will increase ; thus the specific inductive 
capacity, and therefore the refractive index, increases with the frequency of the 
external force ; the effect of the force on the orientation of the molecules will be: 
greatest when the period of the force coincides with that of the molecules. As 
long as the frequency of the force is less than that of the molecules, the external 
field tends to make the molecules set so as to increase the specific inductive capacity 
of the mixture; as soon, however, as the frequency of the force exceeds that of the 
molecules, the molecules, if there are no viscous forces, will all topple over and 
point so as to make the part of the specific inductive capacity due to the molecules 
of opposite sign to that due to the ether. Thus, for frequencies greater than that, 
of the molecules, the specific inductive capacity will be less than unity. When the 
frequency of the force only slightly exceeds that of the molecules, the effect of the 
external field on the molecules is very great, so that if there are a considerable 
number of molecules, the negative part of the specific inductive capacity due to the. 
