etry r<arg 
260 REPORT—1885. 
And we can show that in the value of ©’ it is the 1 in (1 + cos@) which ~ 
comes from the assumed magnetic disturbance, while in ©’ it is the cos 0 — 
in the same term. 
The magnetic disturbance produces no effect in the value of P’. Neglect- — 
ing the magnetic disturbance we arrive at Stokes’s result for the effect of i 
a disturbance X/e®t on the medium, which is used by Rayleigh in the © 
paper on the blue of the sky. : 
Now we may note that the result of the experiments on scattered 
light seems to disprove this hypothesis of Rowland’s as to the necessity of 
considering the two disturbances, for according to him the intensity is the — 
same at all points in the plane zy at the same distance from O, ‘This is 
not true; the intensity varies as sina if a is the angular distance of the — 
point from the axis of z. Again, it is true, of course, that the magnetic — 
disturbance accompanies the electric, but it accompanies itas a consequence, 
If we produce, by some impressed force, a variable electric displacement 
at a point in the medium, and calculate the effect due to this, we have 
done all that is necessary. There will, it is true, be magnetic displace- _ 
ment, but it can be calculated from the electric. 
Rowland’s results do not apply to the case of a wave being propa- 
ted through an aperture, for in this case we have no right to assume 
that the disturbance produced by an element is symmetrical round 
the direction of vibration. We have not a single particle or an indefi- 
nitely small sphere vibrating and sending out its effects in spherical 
waves; we have a state of motion coming in from behind the aperture, — 
and being continually propagated across it at a given point P and at 
time ¢), we must consider the circumstances at any point O of the 
aperture at a time fy such that OP=b(t—f)). For these will be the 
initial circumstances so far as we are concerned; and at this time ft), O- 
has an initial velocity and an initial displacement, Both these require to_ 
be considered in dealing with the question, and we have to adopt Stokes’s! 
method of solution, and we again arrive at his theorems with regard to 
the relation between the direction of vibration and the direction of 
diffraction. : 
§ 2. The electro-magnetic theory, if we accept its fundamental hypo-— 
theses, is thus seen to be capable of explaining in a fairly satisfactory manner 
most of the known phenomena of optics. The great difficulty is, as we 
have said, to account for the properties which the medium must have in 
order to sustain electrical stresses. These consist in an electrostatic field 
of a hydrostatic pressure KR?/87, combined with a tension KR?/4a 
along the lines of force; R being the resultant electrical force, and K” 
the inductive capacity. There will therefore be a difference of pressure 
in different directions in the ether. rs 
Combined with this difficulty there is another of a similar kind, that 
of realising mechanically what electric displacement is, of forming for 
oneself a physical idea of a change of structure in some medium of 
unknown properties which shall obey the laws implied by the various 
equations satisfied by the components of electrical displacement. 
Optical effects are certainly due to changes, periodic in space and 
time, of some properties of a medium which we call the ether. Electro- 
magnetic effects are also due to variations in properties—it may be the 
same as those which give rise to light—of the same ether. When the 
1 On this point reference has already been made, see p. 206. 
