222 The Theory of Immersion. 



settled ; that is to say, there are still some persons who profess not 

 to understand it. But it is now, perhaps, as much settled as, by- 

 discussion, it ever can be settled. For as soon as a controversy has 

 reached the stage at which epithets begin to do duty for arguments, 

 it is a sign that it is about time to let it cease and be judged on its 

 merits, so far, at least, as any profit to science is concerned. Neither 

 should I myself have now added anything more whatever had I not 

 this ulterior question in view, which in some sense necessitates it. 

 For, while so distinguished an observer as Dr. Woodward still pro- 

 fesses to dissent, it might seem unceremonious to pass on without 

 a word to another stage of the question, as assuming the settlement 

 of this. I have, therefore, as a preliminary, to point out why I do so. 

 I will, for this purpose, take a short retrospect of the question in its 

 later stage, not exactly entering on the controversy itself, but only 

 to call attention from a more general point of view to the course it 

 has taken, and the point at which it has now arrived. I therefore 

 deal with it now only as it has been presented by Dr. Woodward. 



One of the things which has probably occurred to the mind of 

 nearly every reader is the wonder how it comes that in a case 

 where the conditions are so few and so definite there could be 

 a controversy at all, and a controversy so difiicult to terminate. 

 Anyone can see how a discussion might go on for ever about, for 

 example, the origin of evil, or the best government for France, or 

 the birth-place of Homer. But in optics the conditions are not 

 only simple but mathematical. The work is always reducible 

 ultimately to combinations of two or three definite laws, which 

 have the precision of a strictly mathematical form ; and a dispute 

 about the result has a kind of resemblance to a dispute as to 

 whether some triangle is three times or only twice as large as 

 some other triangle. In such a case we should conjecture that 

 there must be some simple account of the fact that a difference 

 of opinion could exist at all ; as, for example, whether both parties 

 had really been brought to look at the same triangle. Here, too, 

 the reason is similar and equally simple. The controversy, at the 

 point to which it has now been brought, is not on any one subject 

 or question, but on two different questions alternately. If it could 

 have been kept to one thing it must necessarily have been brought 

 to an issue long since and ended. The difficulty is not at all to 

 meet the theory of Dr. Woodward but to force him to say what 

 his theory is. This is a very old difficulty, and one which, as Locke 

 tells us, no logic ever yet invented can overcome ; because, as he 

 puts it, you cannot eject a vagrant from his dwelling-houae. 



These two different things are, the microscopical objective 

 commonly so called, and the other construction or "machine," 

 put together, not commercially, but, as he himself has informed 

 us, "for the purposes of this controversy." He is required to say. 



