8 PROPAGATION OF LIGHT. 



The light from the source is introduced laterally into the first 

 telescope, through an aperture near the eye-piece. It is then 

 received on a transparent plate, placed between the focus and 

 the eye-glass, and inclined at an angle of 45 to the axis of the 

 instrument. It is thus reflected along the axis of the first 

 telescope, having passed through one of the apertures in the 

 revolving wheel, and is received perpendicularly on the mirror 

 in the focus of the second. It then returns by the same route, 

 and is received by the eye at the eye-glass of the first tele- 

 scope. The distance of the two telescopes in M. Fizeau's ex- 

 periments was 9440 yards. The revolving disc had 720 teeth, 

 and was connected with a counting apparatus which measured 

 its velocity of rotation. The first eclipse took place when the 

 wheel made 12'6 revolutions in a second. With double the ve- 

 locity, the light was again visible; with treble the velocity, 

 there Was a second eclipse, and so on. The mean result of 

 the experiments gave 196,000 miles, nearly, for the velocity 

 of light. 



(12) Let us now proceed to the physical explanation of 

 the foregoing facts. 



We have seen that light travels from one point of space 

 to another in time, and with a prodigious velocity. Now, 

 there are two distinct and intelligible ways of conceiving 

 such a propagated movement. Either it is the same body 

 which is found in different times in distant parts of space ; or 

 there are a multitude of moving bodies, occupying the entire 

 interval, each of which vibrates continually within certain 

 limits, while the vibratory motion itself is communicated in 

 succession from one to another, and so advances uniformly. 

 These two modes of propagated movement may be distin- 

 guished by the names of the motion of translation and the mo- 

 tion of vibration. The former is more familiar to our thoughts, 

 and is that which we observe, when with the eye we follow 



