1 7 2 SEVENTEENTH CENTURY. FT. IIL 



CHAPTER XXI. 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Roemer Velocity of Light Huyghens Cycloidal Pendulum Cor- 

 puscular and Undulatory Theories of -Light Refraction Double 

 Refraction. 



Olaus Roemer measures the Velocity of Light, 1676. 



While Newton was dispersing light in prisms, and finding 

 out its nature, Olaus Roemer, a famous Danish astronomer 

 (born 1644, died 1710), was engaged in something almost 

 as wonderful. He was measuring the rate at which light 

 travels across the sky ! It seems at first as if this would 

 be impossible, but we now know three different ways of 

 accomplishing it ; Roemer's was the first attempt ever made, 

 and his measurement was very near indeed to the truth. 



You will remember that Jupiter has four moons, which 

 move round it as our moon moves round our earth. Three 

 of these moons are so near to Jupiter, and move round it in 

 such, a manner, that they pass through its shadow and are 

 eclipsed every time they go round. Now it became very 

 useful, for certain astronomical reasons, to know exactly when 

 these eclipses happened, and the time of their occurrence 

 was therefore calculated very carefully ever since Galileo 

 first discovered them. There was no difficulty in doing this, 

 and yet, strange to say, the eclipses rarely happened exactly 

 at the right moment. Sometimes they were too early, some- 



