ESSAY ON THE VELOCITY OF LIGHT. 153 
cessively pass before him; when an opening passes, the light of the lamp 
passes through, falls on the mirror, and comes back to the eye in passing 
through the same opening which has hardly been displaced during its passage ; 
soon a tooth presents itself, the light of the lamp is intercepted, and the eye 
sees nothing more; a new opening follows the tooth, and the eye receives the 
light reflected from the mirror, and so on continuously ; the eye, therefore, re- 
eeives the light in an interrupted manner, with a series of extinctions caused 
by the passage of the teeth of the disk. If we increase the velocity of rota- 
tion of the disk these successive apparitions of the light are brought nearer 
and nearer to each other, and as the impression caused by the light on the 
retina lasts some time, about the tenth of a second, after the light has ceased 
to penetrate the eye, when the velocity of the disk is so great that each tooth 
takes less than one-tenth of a second to pass before the eye, the sensation pro- 
duced by the light which has passed an opening has not yet vanished when 
the following opening arrives and renews it; therefore there is no interruption 
in the sensation, and the eye sees the light of the lamp reflected from the mir- 
ror without any intermission; only the intensity of that permanent light which 
it perceives is notably weaker than it would be if the teeth of the disk did not 
pass before the lamp, since the half of the light which it emits in the direction 
of the mirror is intercepted by the teeth which pass successively before it. 
When this continuity in the light is obtained by a sufficient velocity of the 
disk, we are yet far from the particular condition indicated above as giving the 
experiment which can serve to determine the velocity of light; the light which 
passes from the lamp through an opening of the disk can be reflected from the 
mirror almost without loss, and return to strike the eye after having passed the 
same opening; the last portion of this light, that which is emitted when the 
following tooth is on the point of passing before the eye, can alone be inter- 
cepted by this tooth when they return from the mirror to the eye. If we still 
continue gradually to increase the velocity of the disk, these last portions of 
the total light emitted through an opening which are intercepted in their return 
by the following tooth, increase little by little in importance relatively to that 
total quantity of light, and in consequence the intensity of the light received 
by the eye gradually decreases. Finally, when the velocity of the disk has 
so increased that the time employed by its circumference to progress a distance 
equal to the breadth of an opening or of a tooth equals the time the light 
employs to go from the lamp to the mirror and back again to the lamp, the cye 
perceives nothing at all. The light sent by the lamp in the direction of the 
mirror is completely intercepted by the teeth of the disk, as follows: half by 
the anterior faces of these teeth as they pass before the lamp, and the other 
half by the posterior faces of these same teeth after the light, having traversed 
the openings, is reflected back by the mirror. If we increase yet more the 
velocity of rotation of the disk, each tooth passes too rapidly to intercept the 
whole of the returning light which passed through the preceding opening and 
was sent back by the mirror; a portion of that light can therefore pass through 
the opening which follows that tovth and strike the eye, which begins again 
to perceive feebly the image of the lamp in the mirror. 'The velocity of the 
disk still increasing, the intensity of the image increases also until the disk 
turns twice as rapidly as it did the moment the eye perceived no light. Now, 
in fact, all the light which has passed through an opening meets on its return 
from the mirror the following opening which has taken the position of the first, 
and it can enter the eye without the disk intercepting the smallest portion. It 
is easy to sce that, by still adding to the velocity of the disk, the image of the 
lamp, seen by reflection in the mirror, again commences to become faint, again 
to be totally eclipsed, to reappear again, and so on as far as it is possible to 
give increased velocities to the disk. If we observe with care the instant when 
the first of these successive eclipses is completely produced, and if we then 
