| to Galileo.! 
May 13, 1886} 
NATURE 
29 
expenses and 25 francs a day each during its sessions in Rome, 
or other fixed place of meeting. 
The constitution of this Board is admirable, since it secures a 
fair representation of the leading scientific men of Italy. The 
Italian Government does not, it seems, refer such questions 
exclusively to one individual, but endeavours to obtain a con- 
sensus of the scientific opinion of the country. 
Hotel de la Ville, Florence E. Ray LANKESTER 
Fabry’s Comet and Barnard’s Comet 
Nor having seen any mention of the rapid apparent growth 
of the tail of Fabry’s comet, probably some of your readers are 
not aware to how great a length it extended. On April 26 
occurred the first fine night after a very unusual series of over- 
cast ones, and about 14h. G.M.T. I was surprised to see the tail 
reaching up to, or at least to within 1° of, 5 Cassiopeia, a dis- 
tance of 38° from the place given in the ephemeris for the 
nucleus, which was far below the horizon; and the tail would 
doubtless have been’ visible to a greater distance but for the 
brightness of the Milky Way. The following night, about roh., 
it reached at least up to the Cluster in Perseus, a distance also 
of 38° from the predicted position of the nucleus; it was very 
narrow both nights. The next night, which was pretty fine, I 
failed to find any trace of the tail. 
The principal tail of Barnard’s comet is also very narrow : on 
May 1 its length was 44°, as seen with a pair of field-glasses. 
With the telescope this comet had also a faint tail 7/, about 16’ 
long, making an angle of 65° or 70° with the other. 
Sunderland, May 7 T. W. BACKHOUSE 
“« Pumice on the Cornish Coast” 
STEAMER-CINDERS, similar to those referred to by Mr. 
Whitaker in NATuRE for April 29 (p. 604), occur frequently on 
the Falmouth beaches ; but as there seemed to me little proba- 
bility of their being mistaken for pumice, I did not refer to 
the matter in my communication to your columns (April 15, 
Pp: 559): 
Mr. Murray tells me that the pumice I found is felspathic, 
and that from its form and diminished buoyancy it had evidently 
been a long time in the water. The fragment was sent by him 
to Mr. Whitaker, who at once recognised its true character and 
its distinction from the steamer-cinders observed by him on the 
Suffolk coast, one of which he sent to Mr. Murray to satisfy 
him as to their very evident source. H. B. Guppy 
95, Albert Street, Regent’s Park, N.W., May 8 
THE VELOCITY OF LIGHT 
Tf 
[A reinvestigation of this important constant has recently been 
published by Prof. Newcomb. Before we state his methods and 
results we think it well to reproduce the following admirable 
historical notice with which his monograph commences.—ED. | 
HEN it became clearly understood that vision was 
not an immediate perception of objects by the eye, 
but was produced by the passage of an entity called light 
from the object to the eye, the question of the time which 
might possibly be required for this passage became one 
of interest to physical investigators. The first proposal 
for an experimental investigation of this question is due 
t He suggested that two observers, each hold- 
ing a lantern, should be stationed at a distance apart, in 
sight of each other. Each should be supplied with a 
screen, by which he could, in a moment, cover or uncover 
his lantern. One observer should then uncover his lantern 
and the other uncover the other the moment he perceived 
the light from the first lantern. The interval which 
_ elapsed after the first uncovered his light, until he per- 
ceived the light of the second, would be the interval 
required for the light to go and come, plus the time 
required for the second observer to perceive the light and 
make the required movement. This experiment was tried 
by the Florentine Academy, and of course resulted in a 
_* Poggendorff, Geschichte der Phystk, p. 402, wh e i 
he Sage? of the Florentine ineidens i SS aa 
conclusion that the time required was insensible, since we 
now know that it was far below any interval that could 
have been detected by so rude a method. 
It is, however, interesting to notice that, rude though 
this experiment was, the principle on which it was based 
is the same which underlies one of the most celebrated 
methods used in recent times for the attainment of the 
same object. Two very simple improvements which we 
might have imagined the Academicians to make in their 
experiments are these :— 
Firstly, to dispense with the second observer, and in 
his place to erect a mirror, in which the first observer 
could see the image of his own lantern by reflection. The 
time required for the second observer to perceive the 
light and uncover his lantern would then have been 
eliminated from the problem. The interval sought would 
have been that between the moment at which the observer 
uncovered his lamp and the moment at which he perceived 
the reflection. 
Secondly, to use the same screen with which he un- 
covered his own lamp, to cut off the returning ray from 
the distant mirror, and thus obviate the necessity of an 
uncertain estimate of the interval between his muscular 
effort in removing the screen and his perception of the 
return flash of light. If the image was perceived before 
he could cover his own eye with the screen removed from 
the lamp, it would show that the interval of passage was 
less than the time required to make a motion with the 
screen. This interval might have been reduced almost 
indefinitely by having both lines of sight as near together 
as possible. 
Had these improvements been made, the Academicians 
would have had, in principle, Fizeau’s method of measur- 
ing the velocity of light by the toothed wheel, a tooth 
being represented by the screens. To realise the princi- 
ple more fully, the two lines of sight should have been 
rendered absolutely coincident by reflection through a 
telescope. It does not, however, appear that any effort 
to put the question to a severer test was made until the 
subject was approached from a different point of view. 
It was probably considered that the passage was absolutely 
instantaneous, or, at least, that the velocity was above all 
powers of measurement. 
The subject was next approached from the astronomical 
side. In 1676 Roemer made his celebrated communica- 
tion to the French Academy, claiming that observation of 
the eclipses of the first satellite of Jupiter did really prove 
that light required time to pass through the celestial 
spaces.!_ He found 11m. to be the time required for light 
to pass over a distance equal to the radius of the earth’s 
orbit. Dominique Cassini, while admitting that the 
hypothesis of Roemer explained the observed inequality, 
contested its right to reception as an established theory, 
on the ground that the observed inequality might be a 
real one in the motion of the satellite itself. 
Continued observation showed that the time assigned by 
Roemer for the passage of light between the earth and sun, 
or “ the light equation ” as it is briefly called, was somewhat 
too great. In 1809 it was fixed by Delambre at 493°2s., 
from an immense number of observations of eclipses of 
Jupiter’s satellites during the previous 150 years. This 
number has been received as a definitive result with adegree 
of confidence not at all warranted. In 1875, Glasenapp, 
then of Pulkowa, from a discussion of all available 
eclipses of Jupiter’s first satellite between 1848 and 1870, 
showed that results between 496s. and sols. could be 
obtained from different classes of these observations by 
different hypotheses.* 
l Paris Memoirs, tome i. p. 212, and tome x. p. 575. 
2 (bid. tome viil. p. 47. Poggendorff (Geschichte der Physik, p. 656) quotes 
Maraldi as also contesting Roemer’s explanation on the ground that a similar 
inequality should be found depend.ng on the position cf Jupiter in his orbit. 
The ground here taken was quite correct, the enly fallacy being the 
assumption that such an inequality did not exist. 
This paper cf Glasenapp's was published cnly in the Russian language 
as an inaugural dissertation, and in consequence has never become generally 
known 
