CLUSTER TO WHICH THE SUN BELONGS. 



of them which, is observable in these latitudes, to a rigorous 

 telescopic survey, counting the number of individual stars visible 

 in the field of view of a telescope of given aperture, focal length, 

 and magnifying power, when directed to different parts of the 

 firmament. The result of this survey proved that, around two 

 points of the celestial sphere diametrically opposed to each other, 

 the stars are more thinly scattered than elsewhere ; that depart- 

 ing from these points in any direction, the number of stars 

 included in the field of view of the same telescope increases first 

 slowly, but at greater distances more rapidly ; that this increase 

 continues until the telescope receives a direction at right angles 

 to the diameter which joins the two opposite points where the 

 distribution is most sparse ; and that in this direction the stars 

 are so closely crowded together that it becomes, in some cases, 

 impracticable to count them. 



70. The two opposite points of the celestial sphere, around which 

 the stars are observed to be most sparse, have been called the GALAC- 

 TIC POLES ; and the great circle at right angles to the diameter 

 joining these points, has been denominated the GALACTIC CIKCLE. 



This circle intersects the celestial equator at two points, situate 

 10 east of the equinoctial points, and is inclined to the equator at 

 an angle of 63, and, therefore, to the ecliptic at an angle of 40. 



In referring to and explaining the distribution of the stars over 

 the celestial sphere, it will be convenient to refer them to this circle 

 and its poles, as, for other purposes, they have been referred to the 

 equator and its poles. We shall, therefore, express the distance of 

 different points of the firmament from the galactic circle, in either 

 hemisphere, by the terms north or south GALACTIC LATITUDE. 



71. The elaborate series of stellar observations in the northern 

 hemisphere made during a great part of his life, by Sir W. 

 Herschel, and subsequently extended and continued in the south- 

 ern hemisphere by Sir J. Herschel, has supplied data by which 

 the law of the distribution of the stars, according to their galactic 

 latitude, has been ascertained at least with a near approximation. 



The great celestial survey executed by these eminent 

 observers, was conducted upon the principle explained above. 

 The telescope used for the purpose had 18 inches aperture, 20 feet 

 focal length, and a magnifying power of 180. It was directed 

 indiscriminately to every point of the celestial sphere visible in 

 the latitude of the places of observation. 



It was by means of a vast number of distinct observations thus 

 made, that the position of the galactic poles was ascertained. 

 The density of the stars, measured by the number included in 

 each " gauge" (as the field of view was called), was nearly the 

 same for the same galactic latitude, and increased in proceeding 



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