September 15, 1911] 



SCIENCE 



327 



nouncement of common movement in a 

 class of stars of a particular spectral type. 

 The announcement rested to some extent 

 on the work done at the Lick Observatory, 

 much of which has been published in an 

 abbreviated form. But Mr. Campbell, in 

 the Lick Observatory "Bulletin" already 

 quoted, gives reasons why he can not ac- 

 cept the conclusion, which is vitiated, in 

 his opinion, by the existence of a system- 

 atic error in the observations. Now on 

 such a point as this the observer himself is 

 at any rate entitled to a hearing, and is 

 often the best judge. To take proper pre- 

 cautions against systematic errors is the 

 business of the observer, and his efficiency 

 may very well be estimated by his success 

 in this direction — this would be a far safer 

 guide than to judge by results. But some- 

 times such errors, which are very elusive, 

 do not suggest themselves until the obser- 

 vations have been completed, and must be 

 detected from the observations themselves. 

 This, again, is rightly the business of the 

 observer, and the desire to free his observa- 

 tions from such error is a perfectly sound 

 and scientific reason for withholding pub- 

 lication. In the present instance the error 

 is a peculiarly insidious one; and, indeed, 

 we are not even certain that it is an error. 

 It is a possible alternative interpretation of 

 the facts that the stars with Class B spec- 

 trum are in general moving outwards from 

 the sun, and the additional fact that there 

 is a comparatively large volume of space 

 round the sun at present empty of B stars 

 would seem to favor this alternative. But, 

 as already mentioned, the observer himself 

 prefers rather to credit his observations 

 with systematic error which gives a spuri- 

 ous velocity of 5 km. per second to stars of 

 this type. Now it will readily be under- 

 stood how an error of this kind may appear 

 doubled : two vehicles traveling in opposite 

 directions approach or recede from one 



another with double the speed of either; 

 and if one were erroneously supposed to be 

 at rest, the other would be judged to travel 

 twice as fast. In this way the B stars in a 

 particular portion of the sky were judged 

 to be traveling with a common motion of 

 10 km. per second, which would have been 

 a discovery of far-reaching importance if 

 true, but which the observer relegates to 

 the category of systematic errors. 



The illustration will suffice to remind us 

 that the work of the observer is far from 

 being merely mechanical: it demands also 

 skill and judgment — skill in defeating sys- 

 tematic error, and a fine judgment, born of 

 experience, of the success attained. All 

 this is independent of the generalizations 

 which may or may not be arrived at. 

 Bradley 's skill as an observer enabled him 

 to discover the aberration of light and the 

 nutation of the earth's axis; it was en- 

 hanced rather than lessened when he went 

 on to make further observations which, had 

 he lived, would have conducted him to the 

 discovery of the variation of latitude. 

 After his death the world waited more than 

 a century for this discovery to be made, 

 but Mr. Chandler, who played a leading 

 part in it, has declared that Bradley was 

 almost certainly on its track. It would 

 almost seem that an observer is only prop- 

 erly appreciated by another observer. 

 There are doubtless many who, assisted by 

 the knowledge that Bradley's skill had 

 twice previously conducted him to a dis- 

 covery, would be ready to admit the value 

 of his later work, although he did not live 

 to crown it; but how many of these could 

 properly appreciate Bradley without such 

 assistance ? 



I venture to think that the great bril- 

 liance of Newton has dazzled our vision so 

 that we do not see some things quite 

 clearly. 



