310 TRANSACTIONS OF SECTION A. 



form. But Mr. Campbell, in the Lick Observatory ' Bulletin ' already quoted, 

 gives reasons why he cannot accept the conclusion, which is vitiated, in his 

 opinion, by the existence of a systematic 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 precautions against systematic errors is tne 

 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 re- 

 sults. But sometimes such errors, which are very elusive, do not suggest them- 

 selves until the observations 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 observations from such error is a perfectly sound and 

 scientific reason for withholding publication. 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 interpretaton of the facts that the stars with 

 Class B spectrum are in general moving outwards from the Pun, and the addi- 

 tional fact that there is a comparatively large volume of space round the Sun at 

 present empty of B stars would sesm to favour this alternative. Bui, as already 

 mentioned, the observer himself prefers rather to credit his observations with 

 systematic error which gives a spurious velocity of 5 km. per second to stars of this 

 type. Now it will readily be understood how an error of this kind may appear 

 doubled : two vehicles travelling in opposite directions approach or recede from 

 each other with double the speed of either; and if one were erroneously sup- 

 posed 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 travelling 

 with a common motion of 10 km. per second, which would have been a di.s- 

 coverey 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 de- 



ting systematic error, and a fine judgment, born of experience, of the success 

 attained. All this is independent of the generalisations which may or may not 

 be arrived at. Bradley's skill as an observer enabled him to discover the Aber- 

 ration of Light and the Nutation of the Earth's Axis; it was enhanced 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 ob- 

 server is only properly appreciated by another observer. There are doubtless 

 many who, assisted by the knowledge that Bradley's skill had twice previously 

 conducted him to a discovery, 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 pro- 

 perly appreciate Bradley without such assistance ? 



I venture to think that the great brilliance of Newton has dazzled our vision 

 so that we do not see some things quite clearly. 



Had it not been for Newton [writes De Morgan in his ' Budget of Para- 

 doxes,' p. 56] the whole dynasty of Greenwich astronomers, from Flamsteed 

 of happy memory, to Airy, whom Heaven preserve, might have worked away 

 at nightly observation and daily reduction without any remarkable result : 

 looking forward, as to a millennium, to the time when any man of 

 moderate intelligence was to see the whole explanation. What are large 

 collections of facts for? To make theories from, says Bacon; to try ready- 

 made theories by, says the history of discovery; it's all the same, says the 

 idolater ; nonsense, say we ! 



But nothing of this will fit in with what we know of Bradley's work; he 

 discovered aberration, not by any help from Newton, but by accumulating a 

 mass of observations. He had no ready-made hypothesis, or rather he had a 

 wrong one, viz., that the stars would show displacement due to parallax : and 

 after this was proved wrong, ae it was at the very outset, he had nothing in 

 the way of a theory to guide him, and found great difficulty in devising one 



