APRIL 14, 1904] 
NATURE 
561 
tensity of the light is much increased and the im- 
pression upon the eye becomes continuous, but in other 
respects the phenomenon is the same as if there were 
but one spark. 
In order to obtain a measure of the double refrac- 
tion, which is rapidly variable in time, somewhat 
special arrangements are necessary. At the receiving 
end the light, after emergence from the trough contain- 
ing the bisulphide of carbon, falls first upon a double 
image prism, of somewhat feeble separating power, 
so held that one of the images is extinguished when 
the leyden is out of action. The other image would 
be of full brightness, but this, in its turn, is quenched 
by an analysing nicol. When there is double refrac- 
tion to be observed, the nicol is slightly rotated until 
the two images are of equal brightness. This equality 
occurs in two positions, and the angle between 
them may be taken as a measure of the effect. A full 
discussion is given in the paper referred to. 
The finiteness of the angle, which in my experi- 
ments amounted to 12°, is a proof that the light on 
arrival at the CS, still finds it in some degree doubly 
refracting. To obtain the greatest effect the leads 
from the leyden to the deflagrator should be as short 
as the case admits, and the course of the light from 
the sparks to the CS, should not be unnecessarily pro- 
longed. The measure of the double refraction, and in 
an even greater degree the brightness of the light as 
received, are favoured by connecting a very small 
leyden directly with the spark terminals, but the advan- 
tage is hardly sufficient to justify the complication. 
The observations of Abraham and Lemoine bring 
out the striking fact that if the course of the light be 
prolonged with the aid of reflectors so as to delay by 
an infinitesimal time the arrival at the CS,, the oppor- 
tunity to pass afforded by the double refraction is in 
great degree lost, and the angular measure of the 
effect is largely reduced. There is here no change in 
the electrical conditions under which the spark occurs, 
but merely a delay in the arrival of the light. 
The optical arrangements which I found most con- 
venient in repeating the above experiment differ some- 
what from those of the original authors. The sparks 
are taken at a short distance from the polarising nicol 
and somewhat on one side, and in both cases they are 
focused upon the analysing nicol. When the course 
is to be a minimum, the light is reflected obliquely by 
a narrow strip of mirror situated in the axial line, and 
focused by a lens of short focus placed near the first 
nicol. This lens and mirror are so mounted on stands 
that they can be quickly withdrawn, and by means of 
suitable guidance and stops as quickly restored to their 
positions. In this case the distance travelled by the 
light from its origin to the middle of the length of CS, 
is about 30 cm. 
The arrangements for a more prolonged course are 
similar, and they remain undisturbed during one set 
of comparisons. The mirror is larger, and reflects 
nearly perpendicularly ; it is placed upon the axial line 
at a sufficient distance behind the sparks. The light 
is rendered nearly parallel by a photographic portrait 
lens of about 18 cm. focus, the aperture of which 
suffices to fill up the field of view unless the distance 
is very long. In all cases the eye of the observer is 
focused upon the double image of the interval between 
the plates of the CS, leyden. 
The earlier experiments were made at home some- 
what under difficulties. For the blast nothing better 
was available than a glass-blowing foot bellows; but 
nevertheless the results were fairly satisfactory. 
Afterwards at the Royal Institution the use of a larger 
coil in connection with the public supply of electricity, 
and of an automatic blowing machine, gave steadier 
sparks and facilitated the readings. An increase of 
No. 1798, VOL. 69] 
about one metre in the total distance travelled by the 
light reduced the measured angle from 12° to 6°, so 
that the time occupied by light in traversing one metre 
was very conspicuous. } 
It is principally with the view of directing attention 
to the remarkable results of Abraham and Lemoine 
that I describe the above repetition of their experi- 
ment, but I have made one variation upon it which is 
not without interest. In this case the spark is placed 
directly in the axial line and at some distance behind, 
which involves the use of longer leads, and therefore 
probably of a lower degree of instantaneity. The 
additional retardation is now obtained by the insertion 
of a 60 cm. long tube containing CS, between the 
sparks and the first nicol, and the comparison relates 
to the readings obtained with and without this column, 
all else remaining untouched. The difference is very 
distinct, and it represents the time taken in traversing 
the CS, over and above that taken in traversing the 
same length of air. It should be remarked that what 
we are here concerned with is not the wave-velocity 
in the CS,, but the group-velocity, which differs from 
the former on account of the dispersion. 
In the above experiments the leyden, where the Kerr 
effect is produced, is charged comparatively slowly 
and only suddenly discharged. For some purposes the 
scope of the method would be extended if the whole 
duration of the double refraction were made compar- 
able with the above time of discharge. This could be 
effected somewhat as in Lodge’s experiments, where 
a spark, called the B-spark, occurs between the outer 
coatings of two jars at the same moment as the A-spark 
between their inner coatings. The outer coatings re- 
main all the while connected by a feeble conductor, 
which does not prevent the formation of the B-spark 
under the violent conditions which attend the passage 
of the A-spark. The plates of the Kerr leyden would 
be connected with the outer coatings of the jars, or 
themselves constitute the ‘‘outer’’ plates of two 
leydens replacing the jars. RAYLEIGH. 
ENTROPY.* 
N Narvre, April 30, 1903, there is an article entitled 
“ Entropy,” describing at some length the great 
practical use which the engineer now makes of the 
t@ diagram. Engineers very ignorant of mathe- 
matics are able with clearness and certainty to make 
quantitative computations such as used to task the 
powers of mathematicians. The problems so easily 
worked out are very numerous and of a useful, 
interesting character, and mistakes are not easily 
made. On the other side of this question, in a notice 
of Mr. Donkin’s translation of Prof. Boulvin’s ‘‘ The 
Entropy Diagram and its Applications ”’ (NATURE, 
May 4, 1899), it was pointed out that such books were 
doing much harm because they made an illegitimate use 
of the t¢ diagram. Thus I say:—‘‘ Of course we 
may, if we please, say that when steam is released to 
the condenser, we may imagine the whole change as 
occurring in the cylinder itself; only we ought to re- 
member that we are substituting a very simple hypo- 
thetical process for a very complicated reality, which 
has almost nothing in common with it. We ought to 
remember that the very pretty, beautifully complete, 
cyclic t¢ diagrams, which we obtain from childish 
assumptions, may get to be looked upon by students, 
and even by ourselves, as having a real meaning.” 
It is evident that this misuse of the t¢ diagram is 
too prominent in Mr. Swinburne’s mind, and that 
he fails to see the real usefulness of ¢ to engineers. 
1 “ Entropy or Thermodynamics from an Engineer's Standpoint and the 
Reversibility of Thermodynamics.” By James Swinburne. Pp. x+137- 
(Westminster : Archibald Constable and Co.). Price 4s. 6d. net. 
