56 CLASSICS OF MODERN SCIENCE 

 The angle X M E then will be nearly 400 times as great as E S X, 

 which is 5 minutes, i. e., the angular distance travelled by the earth 

 in two hours [the earth traversing almost a degree in a day]. Thus 

 the angle E M X is almost 33 degrees, and likewise the angle M X Y, 

 being 5 minutes greater [than E M X]. 



Now it must be remembered that in this computation it is assumed 

 that the speed of light is such as to consume an hour in passing from 

 here to the moon. But if we assume it to take only a minute of time, 

 then the angle Y X M would amount to only 33 minutes, and if it only 

 takes ten seconds, this angle will be less than six minutes. Now so 

 small an angle is not observable in a lunar eclipse and hence it is not 

 permissible to argue that light is absolutely instantaneous. 



It is rather unusual, we admit, to take for granted a speed 100,000 

 times as great as that of sound, which (following my experiments) 

 travels about 180 toises [about 1150 feet] in a second, or during a 

 pulse-beat. Yet this supposition is not at all impossible, for it is not 

 necessary to carry a body at such speed but only 'for motion to 

 traverse successively from one point to another. 



Hence I do not hesitate in this matter to assume that the passage of 

 light takes time, for on this assumption all phenomena can be ex- 

 plained, while on the contrary supposition none of them can be ex- 

 plained. In fact, it seems to me and to many others as well, that M. 

 Descartes, whose purpose has been to discuss all physical matters 

 clearly, and who has certainly succeeded in this better than any one 

 before him, has written nothing on light and its qualities that is not 

 either hard to understand or even incomprehensible. 



Moreover, this idea that I have propounded as an hypothesis has 

 lately been made a well nigh established fact by that keen calculation 

 of Roemer, whose method I will here take occasion to describe, on the 

 expectation that he will himself in the future fully confirm this theory. 



His method, the same as the one we have just discussed, is astro- 

 nomical. He shows not only that light takes time for its passage, but 

 calculates also its speed and that this must be at least six times as much 

 as the rate I have just given as an estimate. 



In his demonstration he uses the eclipses of the small satellites that 

 revolve around Jupiter, and very frequently pass into his shadow. 

 Roemer's reasoning is this: 



