velocitV of light. 123 



each other, and about 5.\ miles apart, he jihxced 

 two tubes, something like tubes of telescopes, 

 one at each station, looking towards each 

 other, with their axes iu the same straiglit line. 

 One of these tubes, represented at AB, has a 

 branch tube E furnished with lenses, through 

 which is received the light from a radiant point 

 S. This light is reflected by an inclined trans- 

 parent plane mirror fn, forming a bright image 

 of the luminous point at s, which is in the principal focus of the large lens B. 

 The rays being made parallel by this lens, are received at the other station upon 

 the lens C, by which they are brought to a focus upon the surfece of a plani 

 mirror D. Being reflected back by this mirror, they return to the lens B, and once 

 more ibrm a bright image at s, which image may be observed through the trans- 

 parent mirror ?n, by an eye placed at A. The upper side of the tube at F, just 

 in front of the plane mirror, is cut through in order to admit the limb of a wheel, 

 furnished with teeth, to descend so far into it that the image s may be seen be- 

 tween the teeth or cut ofi" by them, according to the position of the wheel. The 

 teeth and the intervening spaces are of exactly equal breadth. By means of 

 connected gear- work a high velocity of rotation may be given to the wheel, 

 while the number of turns per second admits of being ascertained. The 

 velocity may also be retarded by a brake. When the wheel turns slowly 

 the light is intermittent, and the passage of the teeth is perceptible. But when 

 as many as ten teeth pass per second, the light is constant, owing to the dura- 

 tion of the successive impressions upon the eye. By accelerating the movement 

 the brightness of the image may presently be made to fade, in consequence of 

 the interference of the successive teeth with the rays returning from the distant 

 station, after having passed through the last preceding interval between the 

 teeth. As the velocity increases this fading will become an absolute extinction, 

 each tooth in its progress cutting off all the light which passed through the in- 

 terval before it. When this state of things is reached, it is evident that the time 

 occupied in the passage of light to the distant station and back — that is to say, 

 10§ miles — is equal to the time which it takes for a tooth to advance a dis- 

 tance equal to its own breadth. If there are five hundred teeth and five hun- 

 dred intervening spaces, this time will be one one-thousandth part of th.it of a 

 revolution ; and if there are eigliteen revolutions in a second, the absolute time 

 will be one eighteen-thousandth of a second. 



By still further accelerating the velocity of rotation, the light may presently 

 l)e made to reappear; the rays which pass through one opening to the distant 

 station, returning through the next following opening to the eye. When the 

 full brightness is thus restored, the velocity will be found to have been doubled. 

 By carrying the acceleration still further, the light may be a second time eclipsed 

 and a second time restored ; and, in like manner, alternately extinguished and 

 revived, as long as the driving power will allow : the velocities at which the 

 several successive extinctions and revivals occur, constituting a regularly increas- 

 ing arithmetical series. 



VJa thus are enabled to measure the small fraction of a second required for 

 light to pass over twice the distance between the two station? ; and dividing this 

 double distance by this fraction we obtain the velocity of light per second. The 

 result at first obtained by JMr. Fizeau, by means of this apparatus, was about 

 196,000 miles, being in excess of the results by the astronomical methods by 

 one-sixtieth part nearly. 



It is manifest that any mode by which very minute intervals of time can be 

 accurately measured, is capable of being employed as a means of determining 

 the velocity of light. Mr. Wheatstone, in his researches upon the velocity of 

 electricity, employed for this purpose a revolving mirror; and iu 1839, Mr. 



