318 TRANSACTIONS OF SECTION A 



complete proof it is necessary to show that no other hypothesis will suit the facts 

 equally well, and thus we are bound to consider other possible hypotheses even 

 in the direct establishment of one. 



But the method is also old in that it has long been adopted in practice, 

 however partially and unconsciously, by scientific workers of all kinds. When as 

 a boy at school I began to make physical measurements under Mr. J. G. 

 McGregor (now Professor of Physics at Edinburgh) I learnt from him one golden 

 rule : ' Reverse everything that can be reversed.' The crisp form cf the rule may 

 be new to many who have long used it in their work : and its use is simply that 

 of 'multiple hypotheses.' For when the current in a wire is reversed, the hypo- 

 thesis is tacitly made that the effect observed may be due to the direction of 

 the current : and when a measured spectrum photograph is turned round and 

 remeasured, it is an admission of the hypothesis that the direction of measure- 

 ment may be partly responsible for the observed displacements of the spectrum 

 lines. By the various reversals we endeavour, in Professor Chamberlin's 

 words, 'to bring up into view every rational explanation of the phenomenon 

 in hand ' which can be brought up into view in this way. But truly ' no good 

 thing is without its drawbacks,' and one drawback to the recognition of this 

 principle is that, by a process of mental confusion, it seems sometimes to 

 be regarded as a distinct merit in a piece of apparatus that it can be reversed 

 in a large number of ways. It must be remembered that the hypotheses thus 

 examined and ruled out are chiefly instrumental ones superadded to those of 

 Nature : and the latter are already sufficiently numerous, without our ingenious 

 additions. 



The view which I have endeavoured to put before you of the inevitable course 

 of scientific work is that it will depend more and more on the patient process 

 of ' leaving no stone unturned/ It may not be an inspiring view, but it should 

 be at least encouraging, for it follows that no good honest work is thrown away. 

 And it is just this encouragement of which the observer, as opposed to the worker 

 in the laboratory and the mathematician, stands sometimes in sore need. The 

 worker in the laboratory can often clear away his hypotheses on the spot : he 

 can reverse his current tlien and there : but this is often impossible for the ob- 

 server, who can and does reverse his spectrum plate for measurement, but to re- 

 verse the motion of the earth which affected the lines must wait six months : 

 and to reverse also the motion of the star may have to wait six years, or sixty, 

 or sixty thousand. In many cases he must leave the reversal to others, and thus 

 not only can be not test all his hypotheses, but he mav not even be able to formu- 

 late them. His aim cannot therefore be to establish within his lifetime some 

 new law, and his work is not therefore to be appreciated or condemned by his 

 success or failure in this respect. There are truer aims and surer methods of 

 judgment. Something is inevitably lost when we endeavour to express these 

 aims in the concrete; but for the sake of illustration we may say that the true 

 observer is always endeavouring to reach the next decimal place, and is ever on 

 the alert for some new event. Of the pursuit of the next decimal place it is 

 needless t'o say more : the aim is as familiar in the laboratory as in the observa- 

 tory. But I often think that the recognition of new events is scarcely given its 

 proper place in the annals of science, if we have due regard to the consequences. 

 I have protested that in much of bis work the observer cannot be judged by 

 the fruits of his labour, though there is an instinctive tendency to judge in this 

 way : but here is a case where he miuht well be content to be so judged, and 

 yet the consistent award is often withheld. Think for a moment of the very 

 considerable additions to our knowledge which have accrued from the discovery 

 by Professor W. H. Pickering of a Ninth Satellite to Saturn. The discovery led 

 directly to the recognition of the retrograde motion; and to explain this we 

 were led to revise completely our views of the past history of the Solar system. 

 Incidentally it stimulated the search for other new satellites, resulting in the dis- 

 covery of a curious pair to Jupiter and next of the extraordinary Eighth Satel- 

 lite; while i-t was the investigation of the orbit of this curiosity which suggested 

 an eminently successful method of work on Cometary orbits. If we judge scien- 

 tific work by its results we must take into account all this subsequent history 

 in our appreciation of Professor Pickering's achievement. But whether we do 

 so or not is probably a matter of indifference to him, for the true observer is 



