328 EPOCHS IN THE HISTORY OF THE CONTEMPLATION OF 



10th century. Galileo, in the Nuncius Siderius, employs the 

 appellations " stellse nebulosse," and " nebulosse," to de- 

 . note clusters of stars, which, as he expresses it, like " areolse 

 sparsim per sethera subfulgent." As he bestowed no parti- 

 cular attention on the nebula of Andromeda, which is visible 

 to the naked eye but has not yet shewn any stars even 

 under the highest magnifying powers, he regarded all nebu- 

 lous appearances, all his nebulosae, as being like the Milky 

 Way, masses of light formed of closely crowded stars. He 

 did not distinguish between nebula and star, as Huygens 

 did in the case of the nebula of Orion. Such were the first 

 commencements of the great works on nebulae, which have 

 so honourably occupied the first astronomers of our age in 

 both hemispheres. 



Although the 17th century owed its chief splendour, at 

 its commencement, to the sudden enlargement by Galileo 

 and Kepler of the knowledge of the celestial spaces, and, 

 at its close, to Newton and Leibnitz's advances in pure 

 mathematical knowledge, yet it was not without a beneficial 

 influence on the greater part of the physical problems in 

 which we are engaged at the present day. In order not 

 to depart from the character of this history of the contem- 

 plation of the universe, I merely mention the works which 

 exercised a direct and essential influence on general or cosmi- 

 cal views of nature. In reference to Light, Heat, and Mag- 

 netism, we must name first Huygens, Galileo, and Gilbert. 

 When Huygens was occupied with the double refraction of 

 light in crystals of Iceland spar, i. e. with the separation of 

 the pencils of light into two parts, he also discovered, in 

 1678, that kind of polarisation of light which bears his 

 name. More than a century elapsed before the discovery of 



