184 
NATURE 
| Fuly 3, 1873 
friends to testify to the noble simplicity of his dispo- | greatest difficulties of the experiment, for that speed is 
sition, and to the true warmth of his heart, which was 
always open amongst his multifarious and engrossing 
work. 
NEW EXPERIMENTS FOR THE DETERMI- 
NATION OF THE VELOCITY OF LIGHT BY 
M, ALFRED CORNU 
2 ve exact value of the velocity of light is equally in- 
teresting to astronomers and physicists. It is in- 
teresting to astronomers, for it enables us to calculate an 
important and not exactly known number, namely, the 
distance from the sun to the earth, for which cause the 
learned world is looking forward with so much im- 
patience to the passage of Venus on the disc of the sun, 
as the observation of this phenomenon, it is hoped, 
will fill up this chasm, It is interesting to physicists 
likewise, it is evident, but especially since the remarkable 
researches* of Prof. Clerk-Maxwell, who has found an 
unexpected relation between the theories of light and 
electricity. 
M. Alfred Cornu’s experiments, to which we now call 
attention, have for these reasons a great interest. 
The first who busied himself with this difficult question 
was Rcemer, a Dane, at the Observatory of Paris, where 
Picart had called him ; but the observation of the eclipses 
of Jupiter’s satellites, although giving a pretty good value 
of the velocity of light, offers, notwithstanding, some 
causes of error, especially the difference of brightness of 
Jupiter's satellites at their maximum or minimum distance 
from the earth ; and it requires moreover an exact value of 
the diameter of the terrestrial orbit. 
M. Fizeau (1849) showed that it was not necessary to 
employ astronomical phenomena, and that it was possible 
on the surface of the earth to make use of relatively short 
distances, such as four or five English miles. This rather 
bold experiment was much spoken of, He operated be- 
tween Montmartre and Suresnes, near Paris, at a distance 
of about five English miles and a half. 
Léon Foucault, some time after, putting into execution 
a project of Arago, proposed another method founded on 
the revolving mirror of Sir Ch, Wheatstone. The value 
obtained by him, 189,000 miles (298,000 kilometres) was 
made use of by astronomers, who deduced for the paral- 
lax of the sun a number (8”. 86), that is in concordance 
with the best observations of the transit of Venus. 
The number obtained at first by M. Fizeau was higher, 
but it was given by him, who dwelt upon all the diffi- 
culties of such a measurement, with hesitation. 
M. Alfred Cornu left aside Foucault’s method (viz., 
that of the revolving mirror) which is liable to serious 
objections, and employed that of M. Fizeau, although he 
had tried the two methods of experiment at the Poly- 
technic School, where many physicists were able to see 
them. 
M. Fizeau’s method is free from all objection. A 
ray of light is sent between the teeth of a cog-wheel, 
and it is reflected at a great distance, so as to bring it 
back to the point of departure. If the revolving motion 
given to the wheel is sufficiently rapid, the ray on its way 
back meets a tooth, instead of a free passage, and does not 
pass through ; when the speed is double, the ray meets 
the following interval, and passes through again, and so 
forth alternately for increasing rates of revolution. 
Thus the returning ray alternately presents a minimum 
(or an extinction) and a maximum; but the speed of 
rotation (in order to be measured) must be kept constant 
during several seconds in those moments ; it is one of the 
vite See knows that in one ot the last meetings of the British Associa- 
tion Sir William Thomson has estimated them at their real value, 
enormous. Let us add the want of precision in the evolu- 
tion of a maximum or a minimum. 
M. Alfred Cornu has obviated all those difficulties :-— 
1. By giving a speed of rotation not constant but 
increasing or decreasing according to a regular law, 
which he registers by means of electricity; so that he 
easily knows the speed at every moment. 
2. By registering in the same 
manner the exact time in which 
the ray of light disappears and 
appears again : and thus he does 
not observe the instant of maxi- 
mum or minimum, but two in- 
stants which are equally distant 
from the moment that is to be 
determined. 
The various results are traced 
by fine needles that run on a 
sheet of paper covered with 
lamp-black, and rolled round a 
revolving cylinder. If the needles 
remain motionless, they describe 
a helix on the black paper, which 
becomes a straight line when the 
cylinder is unrolled. But these 
points are extremities of arma- 
tures of electro-magnets, and are 
moved when the electricity passes 
through ; and during all the time 
the current passes, the traced line 
is above the level of the normal 
line. 
The annexed sketch shows a 
part of an experiment made in 
the month of July 1872. 
The line a on the right hand 
side represents the increasing 
speed of the wheel; each time a 
cog of the apparatus, in its 
movement of rotation, touched a 
certain wire, the electric current 
had passed through, and devi- 
ated the needle for the time the 
cog was passing (from A to B, 
from C to D). During the time, 
from the beginning of one devia- 
tion to the other (from A to C, 
from C to E, from E to G), 50,000 
teeth had passed. We clearly 
see that these intervals are de- 
creasing, because the speed in- 
creases, 
The median line indicates 
seconds which are sent by an 
electric clock. 
The third line has been ob- 
tained by the observer himself by 
means of a Morse-key ; he made 
the electric current pass during 
the time the light was invisible ; 
P Q/.and sR. S. \ Jihei sketch 
thus shows two extinctions and 
two reappearances of light. 
is the beginning of the experi- 
ment. 
This method, moreoyer, obviates one of the greatest 
difficulties in physical experiments, namely the noting 
down of various numbers, that diverts the observer and 
complicates operations, Furthermore, there remains not 
only the remembrance of the experiment made, but an 
exact, real, and living drawing. 
M, A, Cornu has, moreover, changed the rather large 
Copy of the Automatic 
It Registrations. 
and expensive apparatus of M, Froment for another, 
