J34 A Short History of Astronomy [CH. xii. 



his figures the diameter is about five times the thickness. 

 Further, the Milky Way is during part of its length divided 

 into two branches, the space between the two branches 

 being comparatively free of stars. Corresponding to this 

 subdivision there has therefore to be assumed a cleft in 

 the "grindstone." 



This "grindstone" theory of the universe had been 

 suggested in 1750 by Thomas Wright (1711-1786) in his 

 Theory of the Universe, and again by Kant five years later; 

 but neither had attempted, like Herschel, to collect numerical 

 data and to work out consistently and in detail the conse- 

 quences of the fundamental hypothesis. 



That the assumption of uniform distribution of stars in 

 space could not be true in detail was evident to Herschel 

 from the beginning. A star cluster, for example, in which 

 many thousands of faint stars are collected together in a 

 very small space on the sky, would have to be interpreted 

 as representing a long projection or spike full of stars, 

 extending far beyond the limits of the adjoining portions of 

 the sidereal system, and pointing directly away from the 

 position occupied by the solar system. In the same way 

 certain regions in the sky which are found to be bare of 

 stars would have to be regarded as tunnels through the 

 stellar system. That even one or two such spikes or tunnels 

 should exist would be improbable enough, but as star 

 clusters were known in considerable numbers before Her- 

 schel began his work, and were discovered by him in 

 hundreds, it was impossible to explain their existence on 

 this hypothesis, and it became necessary to assume that a 

 star cluster occupied a region of space in which stars were 

 really closer together than elsewhere. 



Moreover further study of the arrangement of the stars, 

 particularly of those in the Milky Way, led Herschel gradu- 

 ally to the belief that his original assumption was a wider 

 departure from the truth than he had at first supposed ; 

 and in 1811, nearly 30 years after he had begun star- 

 gauging, he admitted a definite change of opinion : 



" I must freely confess that by continuing my sweeps of the 

 heavens my opinion of the arrangement of the stars . . . has 

 undergone a gradual change. . . . For instance, an equal scattering 



