l'.)12] oil Sir William Herschd. 459 



clouds. On the utliLT h;iiul, the spiral and planetary nebulie are more 

 frequent away froui the Milky Way, and these are presumaljly older 

 than the cloudy and flocculeiit nebulas. The shape of the Milky Way 

 seems to resemble a huge millstone or disk of stars, and since it forms 

 a complete circuit in the heavens the sun must lie somewhere towards 

 its middle. It is probable that we look much further out into space 

 along this tract than elsewhere, although it happens that by far the 

 nearest of all the stars — namely, « Centauri— lies in the line of the 

 Milky Way. 



This great congregation of stars is far from uniform in density, 

 for there are places in it where there are but few stars or none at all. 

 Caroline Herschel, writing to Sir John Herschel at the Cape of Good 

 Hope, in 1833, mentions that her brother, when examining the 

 constellation of the Scorpion (which lies at best low down on our 

 horizon), had exclaimed, " after a long awful silence, ' Hier ist 

 wahrhaftig ein Loch im Himmel' " And her nephew, as he said, 

 rummaged Scorpio with the telescope and found many blank spaces 

 without the smallest stir. 



It will explain some of the deductions which Herschel drew from 

 his star-gauges, and will at the same time furnish a good example of 

 his style, if I read a passage from a paper of his written in 1789.* 

 He points out that the sun is merely a star, and, referring to the 

 stars, he continues thus : — 



" These suns, every one of which is probably as of much con- 

 sequence to a system of planets, satellites and comets, as our own 

 sun, are now to be considered in their turn, as the minute parts 

 of a proportionally greater whole. I need not repeat that by my 

 analysis it appears that the heavens consist of regions where suns are 

 gathered into separate systems, and that the catalogues I have given 

 comprehend a list of such systems ; but may we not hope that our 

 knowledge will not stop short at the bare enumeration of phenomena 

 capable of giving us so much instruction ? Why should we be less 

 imiuisitive than the natural philosopher, who sometimes, even from 

 an inconsiderable number of specimens of a plant, or an animal, is 

 enabled to present us with the history of its rise, progress, and 

 decay ? Let us then compare together, and class some of these 

 numerous sidereal groups, that we may trace the operations of 

 natural causes so far as we can perceive their agency. The most 

 simple form, in which we can view a sidereal system, is that of being 

 globular. This also, very favourably to our design, is that which 

 has presented itself most frequently, and of which I have given the 

 greatest collection. 



" But, first of all, it will be necessary to explain what is our idea 

 of a cluster of stars, and by what means we have obtained it. For 

 an instance I shall take the phenomenon which presents itself in 



* Phil. Trans., vol. Ixxix. p. 212. 



