VENUS ON THE SUN'S FACE. 89 



England may be in error. In all such matters it is 

 relative and not absolute error we have to consider. 

 A microscopist would have made a bad mistake who 

 should over-estimate the length of a fly's proboscis by 

 a single hair's breadth : but the astronomer had made 



O ' 



a wonderfully successful measurement of the sun's dis- 

 tance who deduced it within three or four millions of 

 miles of the true value. For it is readily calculable 

 that the error in the estimated relative bearing of the 

 sun as seen from opposite sides of the earth corresponds 

 to the angle which a hair's breadth subtends when seen 

 from a distance of 125 feet. 



The error was first detected when other modes of 

 determining the sun's distance were applied by the 

 skilful astronomers and physicists of our own day. 

 We have no space to describe as fully as they deserve 

 the ingenious processes by which the great problem 

 has been attacked without aid from Yenus. Indeed, 

 we can but barely mention the principles on which 

 those methods depend. But to the reader who takes 

 interest in astronomy, we can recommend no subject 

 as better worth studying than the masterly researches 

 of Foucault, Leverrier, Stone, and Hansen, upon the 

 problem of the sun's distance. 



The problem has been attacked in four several ways. 

 First, the tremendous velocity of light has been meas- 

 ured by an ingenious arrangement of revolving mir- 

 rors ; the result combined with the known time occu- 



