ON OPTICAL THEORIES. 211 
exceeds the critical angle—‘On the Perfect Blackness of the Central 
Spot in Newton’s Rings, and on the Verification of Fresnel’s Formulz for 
the Intensities of the Reflected and Refracted Rays.’! In this paper is 
given the now well-known proof of Arago’s law that light is reflected in 
the same proportion at the first and second surfaces of a transparent plate. 
“On the Colours of Thick Plates,’? and ‘ On the Composition and Resolu- 
ition of Streams of Polarised Light from different sources.’ 3 
In his ‘ Investigations in Optics, with special reference to the 
Spectroscope,’ published in the ‘ Philosophical Magazine’ for 1879 and 
1880, Lord Rayleigh has considered the application of the principles of 
the wave theory to geometrical optics, and the construction of optical 
instruments. A full account of these is given in the article ‘ Optics,’ in 
the ‘ Encyclopedia Britannica.’ 
Professor Stokes’s great paper on Fluorescence‘ is chiefly experi- 
mental. The cause of the phenomena is assigned to the vibrations set up 
by the incident light in the molecules of the fluorescent substance, which 
‘themselves react on the ether and emit the fluorescent light. According 
to Stokes the vibrations in this light are never of shorter period than 
‘those in the incident light; and he in a general way endeavours to 
account for this, and shows that if the force acting on a given matter 
molecule due to a given displacement be proportional to a positive integral 
power of the displacement other than the first, then the amplitude of the 
displacements would involve the period, and there would be a tendency 
to increase the amplitudes of vibrations of lower period than that of the 
incident light, and to decrease the amplitudes in the case of vibrations 
-of higher period than that of the incident light. Thus, in a group of 
disturbed molecules we should expect all possible periods between two, 
the upper corresponding to the refrangibility of the incident light, the 
lower corresponding to the natural period of the molecules. This result, 
known as Stokes’s law, has been the cause of much discussion. Some 
physicists hold that they have found fluorescent substances which con- 
stitute an exception to it, while others,® who have carefully repeated the 
critical experiments, draw conclusions in accordance with the law ; and 
the weight of the evidence is with the latter. 
A general account of the principles of the elastic solid theory was 
‘given in his lectures at Baltimore last year by Sir William Thomson,7 
To these we shall return in the next section. 
§ 2. In concluding this part of the report we may say, then, that 
while the elastic solid theory, taken strictly, fails to represent all the facts 
of experiment, we have learnt an immense amount by its development, 
and have been taught where to look for modifications and improvements. 
We may, I think, infer that the optical differences of bodies depend 
mainly on differences in the density or effective density of the ether in 
those bodies, and not on differences of rigidity. Fresnel’s general theory 
of the cause of reflexion is thus seen to be true, and Green’s theory of 
* Camb. and Dub. Math. Jowrnal, vol. iv.; Math. and Phys. Papers, vol. ii. p. 89. 
? Camb. Phil. Trans. vol. ix. 3 Ibid 
* Stokes, ‘On the Change of Refrangibility of Light,’ Pril. Trans. 
* Lommel, Pogg. Ann. t. 143, p. 159; Wied. Ann. t. iii. viii. x.; Lubarsch, Wied. 
Ann. t. xi. 
§ Hagenbach, Pogg. Ann.; Lamansky, Journal de Physique, t. viii.; Wied. Ann. 
t. vill. and xi. 
” Thomson, Lectures on Molecular Dynamics. 
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