NOVEMBER 19, 1903] 
NATURE 
51 
ferms of interferometer in which the interfering 
pencils are separated and then reunited after reflec- 
tions at a series of plane mirrors is deduced. It is 
explained, further, that with such instruments the 
accuracy of measurement possible with a telescope or 
“microscope can be greatly exceeded, and that, too, 
without serious loss of light. 
The application of interference methods to various 
measurements forms the subject of the remaining 
lectures. 
One of the features of the Edinburgh meeting of 
the British Association in 1892 was Michelson’s paper 
on the application of interference methods to spectro- 
scopic research read before Section A and printed in 
full among the reports. 
Fizeau had years before explained the gradual dis- 
appearance and reappearance of Newton’s rings when 
fermed by sodium light between a flat surface and a 
lens of small curvature as the distance between the 
two is increased. It is due to the fact that the D line 
is double; the ring system seen, therefore, is a com- 
plex one produced by the superposition of the two 
systems due to each line separately. When the bright 
rings of the two systems coincide, the visibility of the 
rings is a maximum; as the distance between the lens 
and plate is increased, the bright rings of the first 
system overlap the dark rings of the second, the 
intensity of the field becomes uniform, and the rings 
cease to be visible. 
Michelson defined the visibility of the ring system 
and showed how it depends on the distribution of light 
in the source; he then proceeded to measure experi- 
mentally the visibility of the rings formed by various 
spectrum lines, and from this to analyse the distribu- 
tion of light in the lines. By a stroke of genius he 
utilised the defects of the ring system to advance our 
knowledge to a surprising extent. Lecture iv. con- 
tains a most interesting account of his work. 
The chapters that follow are no less fascinating; 
thus the next lecture describes the measurements 
undertaken by Michelson at the Bureau International 
des Poids et Mesures at Sévres to determine the re- 
lation between the wave-length of cadmium light and 
the standard metre; cadmium light was chosen 
because of the simplicity of the lines of its spectrum, 
and it was shown that in air at 15° C. and at normal 
pressure the number of waves in a metre is for the 
red ray of cadmium 1553163.5, for the green ray 
1966249-7, and for the blue 2083372.1. The absolute 
accuracy of these results is said to be about one part 
in two millions, the relative accuracy about one part 
in twenty millions. 
In Lecture vii., application of interference methods 
_to astronomy, it is shown how an examination of the 
Visibility curve of a star enables the observer to detect 
double stars which are far too close to be resolved by 
_any telescope, while the last lecture, on the ether, deals 
with a problem which is yet unsolved, the theory of 
aberration. 
The aberration constant, the ratio of the velocity 
of the earth to that of light, is a quantity of the order 
1/10000, and its accurate measurement had proved no 
easy task, Michelson, with the view of solving the 
question whether the earth is at rest or in motion 
NO. 1777, VOL. 69] 
relative to the ether at its surface, undertook a 
measurement which involved the square of this tiny 
quantity, or one part in one hundred millions, and 
carried it out successfully. The result of the experi- 
ment was to show that this relative motion, if it exists 
at all, must be extremely small, and that the ordinary 
explanation of aberration, which assumes that the 
ether remains at rest while the earth moves through 
it without disturbing it, is untenable. The only solu- 
tion of the difficulty yet offered is that due to Lorentz 
and Fitzgerald, who pointed out, independently, that 
the motion of a body through the ether might, on 
certain assumptions as to the connection between ether 
and matter, cause the body to contract in the direc- 
tion of motion, and that this contraction would depend 
on the square of the aberration constant, so that its 
effect might compensate for the effect looked for by 
Michelson. 
In his first lecture the author apologises for using, 
as illustrations of his subject, his own researches. 
‘“T do this,’? he says, ‘‘ because I believe I shall 
be much more likely to interest you by telling what 
I know than by repeating what someone else knows.”’ 
Prof. Michelson has earned our thanks for putting 
some of his knowledge into so attractive a form; he 
will perhaps forgive us if, in closing, we express the 
wish that he will tell us more of what he knows. 
Ree Ge 
ADE ABOUT CAS: 
The Book of the Cat. By Miss F. Simpson. Pp. 
viii+ 380; illustrated. (London: Cassell and Co., 
Ltd., 1903.) Price 15s. net. 
HE ‘cult of the cat ’’ has of late years increased 
to such an enormous extent that there can be 
no doubt as to the need for a thoroughly trustworthy 
and exhaustive account of the various breeds kept in 
this country, together with notices of those of other 
lands. Of this favourable opportunity Miss Simpson 
has taken full advantage in the handsome and beauti- 
fully illustrated volume before us, the exceedingly low 
price of which places it within reach of fanciers in all 
ranks of life. In addition to the description of the 
various breeds kept in this country, the author has 
also given chapters on the feeding, housing, and 
general treatment of cats (derived from her own ex- 
tensive experience), as well as on the management of 
cat-shows; while other chapters by various specialists 
are devoted to foreign breeds, the cat’s place in nature, 
and the diseases of cats and their treatment. The 
book is therefore a compendium of all that relates to 
domesticated cats, and it may be almost said that it 
contains practically all that is worth knowing about 
these animals. 
Perhaps the least satisfactory portion of the book, 
so far as Miss Simpson is concerned, is to be found 
in the opening lines of the first chapter, where we find 
the statement that the origin of the cat has long 
puzzled the learned, and is still a zoological mystery. 
Neither does the second sentence—‘ Historians tell us 
that the feline race came into existence about the same 
