STELLAR CLUSTERS. 



a D. As the visual lines are inclined at greater and greater angles 

 to z A, their length rapidly decreases, as is evident by comparing 

 z A, Z~E, and z F, which explains the fact that while the stars are- 

 as thick as powder in the direction z A, they become less so in 

 the direction z E, and still less in the direction z r, until at the 

 poles in the directions z c and z D, they become least dense. 



On the other side, z B being less than z A, a part of the 

 galactic circle is found at which the stars are more thinly 

 scattered ; but in two directions, z H intermediate between z B 

 and the galactic poles, they again become nearly as dense as in 

 the direction z A. 



This illustration must, however, be taken in a very general 

 sense. No attempt is made to represent the various off-shoots 

 and variations of length, breadth, and depth of the stratum 

 measured from the position of the solar system within it, which 

 have been indicated by the telescopic soundings of Sir W. 

 Herschel and his illustrious son, whose wondrous labours have 

 effected what promises in time, by the persevering researches of 

 their successors, to become a complete analysis of this most mar- 

 vellous mass of systems. Meanwhile it may be considered as 

 demonstrated, that it consists of myriads of stars clustered 

 together : 



' ' A broad and ample road, whose dust is gold, 

 And pavement stars, as stars to us appear ; 

 Seen in the galaxy .that Milky Way, 

 Like to a circling zone powder' d with stars." MILTON. 



The appearance which this mass of stars would present if viewed 

 from a position directly above its general plane, and at a sufficient 

 distance to allow its entire outline to be discerned, was represented 

 by Sir William Herschel as resembling the starry stratum sketched 

 in fig. 10. 



He considered that it was probable that the thickness of this 

 bed of stars was equal to about eighty times the distance of the 

 nearest of the fixed stars from our system ; and supposing our 

 sun to be near the middle of this thickness, it would follow that 

 the stars on its surface in a direction perpendicular to its general 

 plane would be at the fortieth order of distance from us. The 

 stars placed in the more remote edges of its length and breadth he 

 estimated to be in some places at the nine-hundredth order of 

 distance from us, so that its extreme length may be said to be in 

 round numbers about 2000 times the distance of the nearest fixed 

 stars from our system. Such a space light would take 20000 years 

 to move over, moving all that time at the rate of nearly 200000 

 miles between every two ticks of a common clock ! 



15 



