196 ON THE MAGNITUDE OF 



lures us on in our efforts to get better results out of the familiar 

 telescopes and circles which have constituted the standard equip- 

 ment of observatories for nearly a century. Possibly these instru- 

 ments may be capable of indicating somewhat smaller quantities 

 than we have hitherto succeeded in measuring with them, but 

 their limit cannot be far off because they already show the disturb- 

 ing effects of slight inequalities of temperature and other uncon- 

 trollable causes. So far as these effects are accidental, they 

 eliminate themselves from every long series of observations, but 

 there always remains a residuum of constant error, perhaps quite 

 unsuspected, which gives us no end of trouble. Encke's value of 

 the solar parallax affords a fine illustration of this. From the 

 transits of Venus in 1761 and 1769 he found 8"58 seconds in 

 1824, which he subsequently corrected to 8 '5 7 seconds, and for 

 thirty years that value was universally accepted. The first objec- 

 tion to it came from Hansen in 1854, a second followed from Le 

 Verrier in 1858, both based upon facts connected with the lunar 

 theory, and eventually it became evident that Encke's parallax was 

 about one-quarter of a second too small. 



Now, please observe that Encke's value was obtained trigono- 

 metrically, and its inaccuracy was never suspected until it was 

 revealed by gravitational methods, which were themselves in error 

 about one-tenth of a second and required subsequent correction 

 in other ways. Here, then, was a lesson to astronomers who are 

 all more or less specialists, but it merely enforced the perfectly 

 well known principle that the constant errors of any one method 

 are accidental errors with respect to all other methods, and there- 

 fore the readiest way of eliminating them is by combining the 

 results from as many different methods as possible. However, 

 the abler the specialist the more certain he is to be blind to all 

 methods but his own, and astronomers have profited so little by 

 the Encke-Hansen-Le Verrier incidents of thirty-five years ago 

 that to-day they are mostly divided into two great parties, one of 

 whom holds that the parallax can be best determined from a com- 

 bination of the constant of aberration with the velocity of light, 

 and the other believes only in the results of heliometer measure- 

 ments upon asteroids. By all means continue the heliometer 

 measurements, and do everything possible to clear up the mystery 



