W. Harhness — Magnitude of the Solar System. 249 



continually lures us on in our efforts to get better results out 

 of the familiar telescopes and circles which have constituted 

 the standard equipment of observatories for nearly a century. 

 Possibly these instruments may be capable of indicating some-, 

 what smaller quantities than we have hitherto succeeded in 

 measuring with them, but their limit cannot be far off because 

 they already show the disturbing effects of slight inequalities 

 of temperature and other uncontrollable 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 paral- 

 lax affords a tine illustration of this. irom the transits of 

 Venus in 1761 and 1769 he found 8 - 58 seconds in 1824, which 

 he subsequently corrected to 8"57 seconds, and for thirty years 

 that value was universally accepted. The first objection to it 

 came from Hansen in 1854, a second followed from Le Terrier 

 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 trigonometrically, 

 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 per- 

 fectly well known principle that the constant errors of any 

 one method are accidental errors with respect to all other 

 methods, and therefore 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 incident 

 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 combination of the constant of 

 aberration with the velocity of light, and the other believes 

 only in the results of heliometer measurements upon asteriods. 

 By all means continue the heliometer measurements, and do 

 everything possible to clear up the mystery 1 which now sur- 

 rounds the constant of aberration, but why ignore the work of 

 predecessors who were quite as able as ourselves % If it were 

 desired to determine some one angle of a triangulation net 

 with special exactness, what would be thought of a man who 

 attempted to do so by repeated measurements of the angle in 

 question while he persistently neglected to adjust the net ? 

 And yet, until recently astronomers have been doing precisely 



