146 ANNUAL OF SCIENTIFIC DISCOVERY. 



image of the object-glass of each is formed in the focus of the other, 

 furnishes us, in a very simple manner, with the essential condition of a 

 ray of light, which, starting from a point, is reflected at a certain dis- 

 tance so as to return to its starting-point. For this all that is necessary 

 is to place in the first telescope, between the focus and the eye-glass, a 

 transparent glass at an angle of 45 degrees, which sends towards the 

 object-glass the light received obliquely from a lamp or from the sun; 

 and also to place a mirror in the focus of the object-glass of the sec- 

 ond telescope. This arrangement answers perfectly, even when the 

 telescopes are separated to a considerable distance. With telescopes 

 of an aperture of 6 centimetres (2.5 inches) the distance may be 8 

 kilometres (5 miles) without weakening the light too much. We 

 then see a luminous point like a star, formed by the ray of light, 

 which, starting from the focus of the first telescope, and being reflect- 

 ed by the inclined glass through a space of 16 kilometres (10 miles), 

 returns exactly to the same point of departure, traverses the same 

 plate of glass, and finally enters the eye. 



" It is through the point of departure that the teeth of the revolving 

 disk must be passed to produce the effects indicated. The experi- 

 ment is made without any trouble, and the least practised eye per- 

 ceives immediately that, according to the greater or less rapidity of 

 the motion, the point of light shines brightly or is wholly eclipsed, as 

 it meets an open or closed space. Under the circumstances in which 

 I made the experiment, the first eclipse took place when the disk was 

 revolving at the rate of about twelve revolutions and six tenths per 

 second. With a double rapidity the point again shone out, was 

 eclipsed with a triple rapidity, reappeared with a quadruple one, and 

 so on. The first telescope was placed in the cupola of a house sit- 

 uated at Suresnes, and the second one upon the heights of Mont- 

 martre at the approximate distance of 8,633 metres. The disk, having 

 on it 720 teeth, was mounted on wheel-work moved by weights ; a 

 scale furnished the means of measuring the rapidity of the rotation. 

 The light came from a lamp so arranged as to furnish a very brilliant 

 light." 



If we correctly understand the meaning of M. Fizeau, it is evident, 

 in the first place, that the distance of the telescopes, the rapidity of 

 the rotation, and the interval of time which separates the passage of 

 an open from that of a closed space, are known ; secondly, that the 

 meeting of a ray of light with an open or closed space, and conse- 

 quently its reappearance or its eclipse when it has been reflected 

 back after having passed over the double space of 8,633 metres, de- 

 pends solely upon this distance and upon the velocity with which it 

 has been transmitted, and upon the rapidity with which the disk re- 

 volves. Finally, it follows that the only 'unknown quantity in the 

 problem, the velocity of the ray of light, is deduced at once from the 

 two other quantities previously known, namely, the distance passed 

 over and the rapidity of the motion of the disk, joined to the easy ob- 

 servation of the reappearance or the eclipse. 



The repeated experiments made in this manner by M. Fizeau give 

 him for the velocity of light a value differing very little from that as- 



