VELOCITY OE WHITE AND OF COLOURED LIGHT. 
271 
Our method is very like that of M. Fizeatt. We have an observing telescope, in 
the focus of which, and in a plane perpendicular to the axis of which, we have a 
toothed wheel revolving with very great velocity. The light from the sun or from an 
electric or other lamp is condensed by a lens, and reflected by a diagonal reflector 
in the eye-piece, so as to throw an image of the incandescent carbons upon the toothed 
wheel. The light passing between the teeth goes to a distant reflector and returns 
by reflection to the object-glass of the observing telescope, by which the rays are 
brought to a focus in the plane of the toothed wheel at that exact part of the focal 
plane whence the rays had emerged which were capable of striking the distant 
reflector. This point in the focal plane is of course that point at which the observer 
sees the distant reflector (through a small aperture in the diagonal reflector), or at 
which he sees a star of light when the lamp is in action. 
If the double distance to the distant reflector and back were about six miles, we 
know, roughly speaking, light takes about -3T000 second to traverse the double 
distance. Now suppose the wheel to rotate very slowly. We see alternately a tooth 
of the wheel, and a star of light shining in the interval between two teeth. If the 
speed be increased so that more than 10 teeth pass in a second, the persistence of 
visual impressions causes us to be unable to distinguish these alternate phases. We 
see a star of light continuously upon a partially illuminated field. If in - 3 x ^00 second 
a tooth passes into the position previously occupied by a space, then the light which 
passed away through a space to the distant reflector is on its return stopped by a tooth 
and we see nothing but the tooth, while when a space between two teeth is at that 
part of the field of view where the star should appear no star of light is seen, because 
3 T 01511 second ago a tooth occupied that position, and no light could get through to 
go to the distant reflector. If there be 400 teeth in the wheel this speed of revolution 
is 3 T 000 °f a second to one revolution, or 38*75 revolutions a second. At this speed 
of revolution no star of light would be seen, but if the speed be doubled-the light 
passing out through a space to the distant reflector can on its return pass through 
the next space to the eye of the observer, and the light is seen with its full intensity. 
If the speed 38*75 revolutions a second be increased threefold, fivefold, &c., or any 
odd number of times, we have an eclipse of the star. If that speed be increased 
twofold, fourfold, &c., or any even number of times, we have full brightness. 
Fizeatj, and Cornij after him, measured the speed required to produce an eclipse, 
and thence they deduced a value for the velocity of light. Our method, however, 
differs distinctly from theirs in this way: that in place of having a single reflector 
in the distance, we have two at different distances from the observing telescope, but 
nearly in the same line with it, so that the observer in looking through the telescope 
sees two stars side by side separated from each other by a distance of about 25" of arc* 
While the toothed wheel is being rotated with gradually increasing speed, the star 
coming from the more distant reflector (which we call A) is eclipsed before that one 
coming from the nearer reflector (which we call B). As the speed increases A grows 
