ADVANCE OF ASTRONOMY. 193 



show even the slightest trace of optical shifting from 

 the revolution of the earth in its orbit. In about a 

 score of cases, however, small parallaxes have been 

 determined, some certainly (that is, within moderate 

 limits of error), others more or less precariously." 



Dr. Fison notes that for forty years after Bessel's 

 discovery the record is chiefly one of accumulation 

 of experiences ; " and when in 1881 Dr. Gill and 

 Dr. Elkin commenced a series of observations at 

 the Cape of Good Hope, the parallaxes of not more 

 than half a dozen stars had been detected with cer- 

 tainty. Since that date, however, parallax hunters 

 have been better rewarded, though up to the present 

 time (1898) it is doubtful whether success has been 

 achieved in more than fifty instances." f 



The distances of the stars whose remoteness is 

 measurable are so enormous that they produce almost 

 no impression on the ordinary mind. 



" It follows," said Bessel, " that the distance of 

 61 Cygni from the sun is 657,700 times the half 

 diameter of the earth's orbit. The light from the 

 star takes ten years to traverse this enormous dis- 

 tance. It is so vast, that though it may be conceived, 

 it cannot be visualised. All attempts to realise it, 

 fail either because of the size of the unit of measure- 

 ment or because of the number of repetitions of the 

 unit. The distance which light traverses in a year 

 is not more realisable than that traversed in ten 

 years. Or if we choose a realisable unit, such as 

 the distance of 200 miles which a locomotive (bicycle, 

 we should say) travels in a day, it would require 

 68,000 millions of such daily journeys, or about 200 



* A. M. Clerke. History, 1885, p. 48. 

 t Recent Advances in Astronomy, 1898, p. 7. 



