258 REPORT—1885. 
concerned, its properties are those of an elastic solid in which the elec- 
trical displacement f is given by 
ee d (= dn *) 
The objection that it is impossible to maintain a continuous molecular 
rotation in an elastic solid may be made to this analogy. It seems, how- — 
ever, possible that, as suggested by Professor Stokes when considering 
the problem of aberration, the ether may behave as a perfect fluid for all 
motions involving more than a very small relative displacement of its 
parts, while for such small displacements as are contemplated in the 
theory of light it has, in a dielectric, an appreciable rigidity. In a con- 
ductor the effects of this rigidity, if it exist, are masked by the more 
powerful effects of the viscosity. The fluid is no longer perfect. 
Chapter [V.—Rowtanp’s THErory Or THE PROPAGATION OF PLANE WavES. 
p 
§ 1. The propagation of waves of electro-magnetic disturbance from 
a given source has been recently very fully considered by Professor 
Rowland,! and we proceed to give some account of his paper. 
Rowland considers very generally the solution of the equations— 
(ad (ee 
Te =V*v’F,. : : ; i here 
etc., and allied equations given by the system 
_ dH, = dG, 
F on41 ay <a , ‘ . (28) 
so that F, G, H satisfy the solenoidal condition. He puts 
Fo = C,p"®_n41) eae earn 
® _ (41) being a spherical harmonic, and C,, a function of p. 
He finds 
PC, + 2 (a _ ib) os met) C,, = 0 ° , e (29) 
dp? 
whence f 
ei yeti nm (n® — 1°) (n + 2) ae a 
0,=0{1- 9°40 48 + (30) 
where c =a — ib. 
He then takes, as a special solution, 
ios n de® _,, c(p — Vt) 
Fo—iC, 5 ae ; ; : me 58) 
and a¢ n=l { AD ig to-»\ c(p —Vt) 
F, =cC_ 1p y— & aaa e Dect @rs2) 
etc., and treats the case of symmetry round the axis of w, for which 
ait eoken "Os 
—n pn! ? 
Q,-1 being a zonal harmonic with the axis of x for axis. 
1 Rowland, American Journal of Mathematics ; Phil. Mag. June, 1884. 
