156 ESSAY ON THE VELOCITY OF LIGHT. 
‘Tn observing the image reflected by the mirror attached to the third piece of wheel-work, 
the effects observed should be identical with those which should be given by a revolving 
mirror making 3,000 turns per second. From this moment the success ef the projected 
experiment was placed beyond doubt. It was only to be regretted that, by the three suc- 
cessive reflections from three different mirrors, the light necessarily experienced a considera- 
ble loss in intensity. Jt was, therefore, desirable to arrive at the result by a single reflection; 
and it is to this that the expenments which I am going to relate seem to lead. 
“Tn his investigations into the causes which prevented us revoiving a mirror more than 
1,000 turns per second, M. Bréguet proposed to relieve the last axis of the weight of the 
mirror with which it was charged, to turn the axis alone; and he succeeded, not without 
surprise, in giving to this axis 8,000 turns per second. ‘The obstacle which prevented us 
giving the same axis, when it carried the murror, a velocity greater than 1,0U0 tarns per 
second appeared evident. It was, one would think, the resistance of the air. I myself. 
thought of the existence of that cause, and all our though s were directed to the means of 
revolving the mirror ina vacuum. We immediately constructed a metallic receiver destined 
io hold the revolving apparatus. This receiver had several apertures, of which one was to 
give entrance to the rays of light after having traversed the two columns of air and of liquid. 
Before the others were to be the ebjectives of the telescopes, with which to observe the 
rays reflected by the rotating mirror, the necessary communications were established by 
means of stufling-boxes between the apparatus and the driving weight. A special tube put 
the interior of the receiver in-communicaion with an air-pump. 
‘All was arranged and placed upon a stone column in the meridian room of the observa- 
tory. It only remained to make the observation. * 4 si et rs > * (The 
murror, contradictimg all our anticipations, turned hardly any faster in the vacuum than in. 
the air. This circumstance again showed the truth of the proverb, ‘‘ Le mieux est ’ennemi 
du bien:” (Better is the enemy of good enough.) It was necessary to think of return- 
ing to the first apparatus composed of three pieces of wheel-work and of three separate 
mirrors, the apparatus which Lt had given up only to obtain a greater intensity in the 
reflected ray. 
“‘T was convinced of: the necessity of going back to the first method of experiment at the 
time when my enteebled sight would not allow me to undertake it. My pretensions, there- 
fore, ought to be limited to having proposed the problem, and of having given the certain 
means of solving it. These means may, during its accomplishment, experience modifica- 
tions, which will render them applicable, with more or less facility, without changing their 
essential character.” 
After some details, which it is useless to give here, Arago adds : 
‘*As to myself, if I have delayed a long time the realization of that which I had announced 
that has been owing in large part to the obligations which M. Bréguet, my collaborator, had 
contracted with the government for the supply of electric telegraphs, and to the desire that I 
had to operate, as I have already said, with a mirror making 8,0U0 turns per second. 
‘* Probably, also, I may remain content with the thought that no one will execute, with- 
out my authorization, an experiment founded on principles and methods of execution which 
J have exposed to the world in their most minute details. 
**M. Bessel, after my publication in the Compie fendu, announced to me that he had 
thought of a modification of my apparatus composed of three successive pieces of wheel- 
work, each carrying a mirror. He receives the image reflected by the first rotating mirror 
not upon a second revolving mirror, but upon a fixed mirror, which sends the ray back to 
the first mirror. After this second reflection, the rays fall again upon a fixed mirror, from 
which they are reflected a third time to the turning mirror, &c. It is after the last reflection 
from the single revolving mirror that M. Bessel proposes to measure the angular departure 
of the ray. This method, more simple than the one i proposed, in so far as it required only 
one piece of wheel-work, had the very grave inconvenience of diminishing much more the 
light, since he had more reflections from the mirrors than in the other method. * * * 
*“M. Silbermann, without knowledge of the prior communication of M. Bessel, made me 
a proposition similar to that of the illustrious observer of Koenigsberg. 
« * ¥ * ‘a * * a * * # * 
‘Things were in this state when M. Fizeau determined by his so ingenious experiment 
the velocity of light in the atmosphere. Thet experiment was not indicated in my memoir. 
‘The author, therefore, had the right to make it without exposing himself to the slightest 
reproach for want of due consideration of the rights of others. 
“As to the experiment on the comparative velocity of light in a liquid and in air, the 
author wrote to me: ‘I have not yet made any attempt in that direction, and I will not 
occupy myself with it but on your formal invitation, ‘his loyal reserve could only add to 
the esteem with which the character and the works of M. Fizeau had inspired me, and I 
willingly authorized M. Bréguet to lend him one or several of my rotating mirrors. 
‘““M. Foucault, whose inventive genius is well known to the Academy, came also to 
inform me of the desire he had to submit to the test of experiment a modification which he 
had devised in my apparatus, , 
