58 CLASSICS OF MODERN SCIENCE 



The motion of Jupiter in its orbit while the earth passes from B to 

 C or from D to E has been taken into consideration in Roemer's cal- 

 culation, where it is also proved that these inequalities cannot be 

 caused by any irregularity or eccentricity in the movement of the 

 satellite. 



Now if we consider the enormous size of this diameter K L [the 

 earth's orbit] which I have estimated to be about 24,000 times that of 

 the earth, we get some comprehension of the extraordinary speed of 

 light. 



Even if K L were only 22,000 diameters of the earth, a speed trav- 

 ersing this distance in 22 minutes would be equal to the rate of a 

 thousand diameters a minute, i. e., 16 2-3 diameters a second (or a 

 pulse-beat) which makes more than 1,100 times 100,000 toises, since 

 one diameter of the earth equals 2,865 leagues, of which there are 25 

 to the degree, and since in accordance with the very precise calculation 

 made by M. Picard in 1609 under orders from the king, each league 

 contains 2,282 toises. 



As I stated before sound moves only 180 toises per second. Hence 

 the speed of light is over 600,000 times as great as that of sound, 

 which, however, is very different from being instantaneous, — it is the 

 difference between any finite number and infinity. The theory that 

 light movements are propagated from point to point in time being thus 

 demonstrated, it follows that light moves in spherical waves, as does 

 sound. 



But if they are alike in this regard, they are unlike in others, as in 

 the original cause of the motion that transmits them, the medium 

 through which they move, and the manner in which they are trans- 

 mitted in it. 



We know that sound is caused by the rapid vibration of some body 

 (either as a whole or in part), this vibration setting in motion the ad- 

 joining air. But light movements must arise at every point of the 

 luminous body, otherwise all the various parts of the body would not 

 be visible. This fact will be clearer from what follows. 



In my judgment, this movement of light-giving bodies cannot be 

 more satisfactorily explained than by supposing that those that are 

 fluid, e. g., a flame, and probably the sun and stars, consist of particles 

 that float about in a much rarer medium, that sets them in violent 

 motion, causing them to strike against the still more minute particles 



