438 
NATURE 
[JuNE 27, 1912 
executed in or across the plane of polarisation. Max- 
well dissipated the controversy when on his electro- 
magnetic theory of light he showed that both were 
present, the elastic vibrations taking place across the 
plane, and the magnetic ones in it. 
To-day, and ever since Maxwell propounded the 
electromagnetic theory, the main interest has been 
transferred to the question how the ether is related 
to electricity and to ponderable matter, and whether 
the motion of matter in space affects or is affected 
by the ether. Is it a fact that the zther is stagnant, 
fixed, ‘‘ while the molecules constituting the earth and 
all other material bodies flit through it without pro- 
ducing any flow in it’’?’ Or is the zther speeding 
along with the earth and the whole solar system in 
headlong and enormous flight? That singular doc- 
trine, now in fashion, called ‘‘The Principle of Rela- 
tivity,’’ invites us first to deny that we can ever detect 
or measure the absolute velocity of the earth in space, 
and then to admit that, therefore, since we cannot 
regard the ether as filling space or fixed in it, we 
must abolish the notion of the zther as a conveying 
medium, and must explain the finite velocity of light 
in some other way depending on electromagnetic 
principles inherent in the light impulse, and expressed 
in terms of coordinates the origins of which are to 
be only relatively, and not absolutely, fixed. With- 
out pursuing these anarchical ideas, we may remark 
that for all useful purposes it suffices to admit that 
no terrestrial optical phenomena have any relation to 
the direction of the earth’s motion through the 
universe. 
As for the relation between matter and ether, 
while for clarity of thought we must frame some idea 
of the connection between them, we may accept Sir 
Joseph Larmor’s dictum that ‘‘Matter may be, and 
likely is, a structure in the zether, but certain ether 
is not a structure made of matter.’”’ His view that 
“the motion of matter does not affect the quiescent 
zther, except through the motion of the atomic 
electric charges carried along with it,’ is, of course, 
bound up with the further conception that the zther 
is a plenum in which “vortices or other singularities 
of: motion and strain” are the nuclei of which matter 
consists.* 
SPECTACLE OPTIcs. 
The fixing of two lenses together to form a pair 
probably dates from the thirteenth century, but 
history is obscure. Raphael, in 1517, painted Pope 
Leo X. wearing concave spectacles. But not all 
pictures are good as evidence, for there is, or was, 
in the Chiesa de’ Ognissanti, in Florence, a picture 
attributed to Sandro Botticelli, depicting St. Jerome 
in his cell, with a pair of spectacles beside him. This 
does not prove that spectacles existed in the fourth 
century; and the presence of the spectacles may be 
as great an anachronism as in another picture of 
the same Saint is the presence, on the wall of the 
cell, of a pendulum clock. Coming down to the pre- 
sent day, few persons probably are aware of the rapid 
rate at which that branch of the subject is develop- 
ing into a severe scientific study. Perhaps they 
think that the only progress in spectacle-making has 
been the introduction of lighter spectacle frames or 
ingenious dodges for grinding bifocal glasses, or for 
fusing one kind of glass into another for a bifocal 
lens, or for grinding toric lenses. This would be 
quite a mistake. It may be that the teaching in the 
medical schools has remained much as it was; but the 
training of spectacle opticians to deal with the 
problems of astigmatism, both of eyes and of lenses, 
° Larmor, ‘ Aether and Matter” (1900), p. 167. 
4 Particular reference may be made to Sir Joseph T-armor's ‘‘ Ether and 
Matter” (1900), and to Prof. E. T. Whittaker’s ‘‘ History of the Theories of 
A&ther and Electricity,” 191c. 
NO. 2226, voL. 89] 
has taken great strides, and under the stimulus of 
the system of certification by the Spectacle Makers’ 
Company and of other optical bodies is assuming an 
important development. ; 5 
Apart from actual practice, an _ exceedingly 
important advance in theory has been initiated by the 
genius of Allvar Gullstrand. In the year 1903 he 
pointed out that the centre of rotation of the eyeball 
does not coincide with the nodal point, which is its 
optical centre. It is, in fact, from 2 to 3 millimetres 
behind it, and therefore in all those uses which the 
eye makes of its power of turning about in its socket 
the mathematical treatment which assumes it to be 
fixed is inadequate. The assumptions of the Gauss 
system are no longer fulfilled, and modifications have 
been introduced. For precise work this affects the 
efficiency of spectacle lenses and introduces new 
sources of aberration. For this reason spectacles 
should be so designed that the particular point at 
which they are corrected for radial astigmatism should 
lie at the centre of rotation of the eye. 
One other point in spectacle optics needs attention. 
Thirty years or more have passed since British 
opticians ceased. to denominate their lenses in terms 
of inches of focal length, and adopted the dioptric 
system of numbering, in which a lens having a focal 
length of 1 metre is described as having a power 
of one diopter, and a lens of twice that power as of 
two diopters. The diopter, the international unit of 
lens power, was adopted in 1875 on the proposal of 
Monoyer at the Brussels Conference. Nearly thirty 
years ago it was pointed out that the diopter, being 
the reciprocal of a length, is in reality a unit of 
curvature, and may be applied to express curvatures 
of wave-fronts and of surfaces, as well as the power 
of a lens, which is, in fact, merely the expression of 
the convergency which it imposes on the light passing 
through it. 
OprticaL EpucaTion. 
To the optical industry as a whole the question of 
the scientific training of young men who shall here- 
after become technical leaders and pioneers is a very 
serious one, in view of the stress of the times. Men 
are wanted who can undertake mathematical calcula- 
tions with a first-hand practical knowledge how these 
calculations are applied in the design of instruments, 
and who have a thorough acquaintance with the 
whole range of optics. That training at present they 
cannot acquire at any of the universities. It is a 
melancholy fact that now, when this need is sorest, 
the pursuit of optics at our universities and colleges 
is in a deplorable state. Except in the Northampton 
Polytechnic, and one or two other institutes, the study 
of optics for its own sake is entirely ignored. Not 
one of the universities of Great Britain has created a 
chair of optics, though there are professorships and 
extensive laboratories for electrical engineering, for 
metallurgy, and for various branches of technical 
chemistry. In the universities and colleges the only 
people who are learning optics are merely taking it 
as a part of physics for the sake of passing examina- 
tions for a degree, and care nothing for the applica- 
tions of optics in the industry. They are being taught 
optics by men who are not opticians, who never 
ground a lens or calculated even an achromatic 
doublet, who never worked with an ophthalmoscope 
or measured a cylindrical lens. 
Again and again, as might be demonstrated by 
many instances, advances in optics have come about 
through the association of the highly trained mathe- 
matician with the practical workman, and most 
effectually when these are combined in one individual. 
But where is England to look for the training up of 
| such men? For twenty-five years some of us have 
