[72 SEVENTEENTH CENTURY. pt. hi. 



CHAPTER XXI. 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Roemer measures the Velocity of Light— Newton's Corpuscular Theory 

 of Light — Undulatory or Wave-theory proposed by Huyghens — 

 Invention of Cycloidal Pendulums by Huyghens — Discovery of 

 Saturn's Ring — Sound caused by Vibration of Air — Light by Vibra- 

 tion of Ether — Reasons why we see Light — Reflection of Waves of 

 Light — Cause of Colour — Refraction explained by the Undulatoiy 

 Theory — Mr. Tylor's Illustration of Refraction — Double Refraction 

 explained by Huyghens -Polarisation of Light not understood till 

 the nineteenth century. 



Olaus Eoemer measures tlie 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 17 10), 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 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 



