SECT. XXXVI. DISTANCES OF FIXED STARS. 387 



penetrates through the starry stratum, and beyond it. Sometimes 

 we look through a sheet of stars nearly of the same size, of no 

 great thickness compared with their distance from us, and not 

 unfrequently there is a double stratum, one 'of large stars spread 

 over another of very small ones. 



The most southerly of the two streams of stars which form 

 the Milky Way in this part of the firmament maintains an 

 unbroken course of extreme brilliancy, containing some of the 

 finest clusters of stars in the heavens. One round y Sagit- 

 tarii is an intense aggregate of stars, in some parts of which 

 they are so crowded as to exceed enumeration ; at a very mode- 

 rate estimate Sir John Herschel thinks this group cannot contain 

 fewer than a hundred thousand stars. Other two groups be- 

 tween the constellations of the Shield and Ophiuchus stand out 

 like promontories of intense brilliancy in the dark space that 

 separates the starry streams of the Milky Way. 



The distance of the fixed stars is too great to admit of their 

 exhibiting a sensible disc, but they must be spherical if gravita- 

 tion pervades all space, as there is every reason to believe it does. 

 With a powerful telescope the stars are like points of light : their 

 occultations by the moon are therefore instantaneous. Their 

 twinkling arises from sudden changes in the refractive power of 

 the air, which would not be sensible if they had discs like planets. 

 Thus nothing can be known of their distance from us or from 

 one another by their apparent diameters. Although from the 

 appearance of the stars no inference can be drawn as to their 

 distance, yet among the multitudes in the heavens a few are 

 found near enough to exhibit distinct parallactic motions arising 

 from the revolution of the earth in its orbit, from whence their 

 distance from the sun has been computed : a Centauri, the 

 brightest star in the southern hemisphere, is a very remarkable 

 instance. Professor Henderson at the Cape of Good Hope deter- 

 mined its parallax to- be 1" by a series of observations on its posi- 

 tion at opposite periods of the year, that is, from opposite points 

 in the earth's orbit. The result was afterwards confirmed by Mr. 

 Maclear, who found the amount to be 0".913. The difference 

 between the two is wonderfully small, considering the many un- 

 avoidable sources of error in the determination of such minute 

 quantities (N. 230). 



Since no star in the northern hemisphere has so great an amount 



8' 2 



