152 THE HISTORY OF SCIENCE 



competent to explain on mechanical principles the 

 development of the solar system from the point at 

 which he undertook it. In his later years he desired 

 that the misleading anecdote should be suppressed. 

 So far was he from self-sufficiency and dogmatism 

 that his last utterance proclaimed the limitations of 

 even the greatest intellects : " What we know is little 

 enough, what we don't know is immense " (Ce que 

 nous connaissons est pen de chose, ce que nous ig- 

 norons est immense). 



Sir William Herschel's observations, extended 

 over many years, confirmed both the nebular hypoth- 

 esis and the theory of the systematic arrangement of 

 the stars. He made use of telescopes 20 and 40 feet 

 in focal length, and of 18.7 and 48 inches aperture, 

 and was thereby enabled, as Humboldt said, to sink 

 a plummet amid the fixed stars, or, in his own 

 phrase, to gauge the heavens. The Construction of 

 the Heavens was always the ultimate object of his 

 observations. In a contribution on this subject sub- 

 mitted to the Royal Society in 1787 he announced 

 the discovery of 466 new nebulae and clusters of 

 stars. The sidereal heavens are not to be regarded 

 as the concave surface of a sphere, from the center 

 of which the observer might be siipposed to look, 

 but rather as resembling a rich extent of ground 

 or chains of mountains in which the geologist dis- 

 covers many strata consisting of various materials. 

 The Milky Way is one stratum and in it our sun 

 is placed, though perhaps not in the very center of 

 its thickness. 



By 1811 he had greatly increased his observations 

 of the nebulae and could arrange them in series differ- 



