8o 



SCIENCE. 



[Vol. XII. No. 2i 



safe to say, would include illustrations from other branches of 

 science, as well as my own. 



But — and here I ask pardon if I speak of myself — I have been 

 led to review the labors of other searchers from this standpoint, be- 

 cause I had first learned, out of personal experience, that the most 

 painstaking' care was no guaranty of final accuracy ; that to labor 

 in the search for a truth with such endless pains as a man might 

 bestow if his own salvation were in question did not necessarily 

 bring the truth ; and because, seeking to see whether this were the 

 lot of other and greater men, I have found that it was. and that, 

 though no one was altogether forsaken of the truth he sought (or, 

 ■on the whole review of his life as a seeker, but might believe he had 

 advanced her cause), yet there was no criterion by which it could 

 be told at the time, whether, when after long waiting there came in 

 view what seemed once more her beautiful face, it might not prove, 

 after all, the mockery of error ; and probably the appeal might be 

 made to the experience of many investigators here with the ques- 

 tion, " Is it not so ? " 



What then ? Shall we admit that truth is only to be surely found 

 under the guidance of an infallible church ? If there be such a 

 church, yes ! Let us, however, remember that the church of science 

 is not such a one, and be ready to face all the consequences of the 

 knowledge that her truths are put forward by her as provisional 

 only, and that her most faithful children are welcome to disprove 

 them. 



What then, again? Shall we say that the knowledge of truth is 

 not advancing ? It is advancing, and never so fast as to-day ; but 

 the steps of its advance are set on past errors, and the new truths 

 become such stepping-stones in turn. 



To say that what are truths to one generation are errors to the 

 next, or that truth and error are but different aspects of the same 

 thing to our poor human nature, may be to utter truisms ; but truisms 

 which one has verified for one's self out of a personal experience 

 are apt to have a special value to the owner ; and these lead, at any 

 rate, to the natural question, " Where is, then, the evidence that we 

 are advancing in reality, and not in our own imagination ? " 



There are many here who will no doubt heartily subscribe to the 

 belief that there is no absolute criterion of truth for the individual, 

 and admit that there is no positive guaranty that we, with this 

 whole generation of scientific men, may not, like our predecessors, 

 at times go the wrong way in a body, yet who believe as certainly 

 that science as a whole, and this branch of it in particular, is ad- 

 vancing with hitherto unknown rapidity. In asking to be included 

 in this number, let me add that to me the criterion of this advance 

 is not in any ratiocination, not in any a priori truth, still less in the 

 dictum of any authority, but in the undoubted observation that our 

 doctrine of radiant energy is reaching out over nature in every 

 •direction, and proving itself by the fact that through its aid nature 

 obeys us more and more ; proving itself by such material evidence 

 as is found in the practical applications of the doctrine, in the 

 triumphs of modern photography, in the electric lights in our streets, 

 -and in a thousand ways which I will not pause to enumerate. 



And here I might end, hoping that there may be some lessons 

 for us in the history of what has been said. I will venture to 

 ask the attention to one more, perhaps a minor one, but of a practi- 

 cal character. It is that in these days, when the advantage of 

 organization is so fully recognized, when there is a well-founded 

 hope that by co-operation among scientific men knowledge may be 

 more rapidly increased, and when in the great scientific depart- 

 ments of government and elsewhere there is a tendency to the 

 formation of the divisions of a sort of scientific army, — a tendency 

 which may be most beneficially guided, — that at such a time we 

 should yet remember, that, however rapidly science changes, human 

 .nature remains much the same ; and (while we are uttering truisms) 

 let us venture to say that there is a very great deal of this human 

 nature even in the scientific man. whose best type is one nearly as 

 'Unchanging as this nature itself, and one which cannot always advan- 

 tageously be remodelled into a piece of even the most refined bu- 

 reaucratic mechanism, but will work effectively only in certain ways, 

 -and not always at the word of command, nor always best in regi- 

 ments, nor always best even under the best of discipline. 



Finally, if I were asked what I thought were the next great steps 

 to be taken in the study of radiant heat, I should feel unwilling to at- 



tempt to look more than a very little way in advance. Immediately 

 before us, however, there is one great problem waiting solution. I 

 mean the relation between temperature and radiation ; for we know 

 almost nothing of this, where knowledge would give new insight in- 

 to almost every operation of nature, nearly every one of which is 

 accompanied by the radiation or reception of heat, and would en- 

 able us to answer inquiries now put to physicists in vain by every 

 department of science, from that of the naturalist as to the enigma 

 of the brief radiation of the glow-worm, to that of the geologist 

 who asks as to the number of million years required for the cooling 

 of a world. 



When, however, we begin to go beyond the points which seem, like 

 this, to invite our very next steps in advance, we cannot venture to 

 prophesy ; for we can hardly discriminate among the unlimited possi- 

 bilities which seem to open before a branch of knowledge which deals 

 especially with that radiant energy which sustains, with our own 

 being, that of all animated nature, of which humanity is but a part. 

 If there be any students of nature here, who, feeling drawn to labor 

 in this great field of hers, still doubt whether there is yet room, 

 surely it may be said to them, " Yes, just as much room as ever, as 

 much room as the whole earth offered to the first man ; " for that field 

 is simply unbounded, and every thing that has been done in the 

 past is, I believe, as nothing to what remains before us. 



The days of hardest trial and incessant bewildering error in 

 which your elders have wrought seem over. You " in happier ages 

 born," you of the younger and the coming race, who have a mind 

 to enter in and possess it, may, as the last word here, be bidden to 

 indulge in an equally unbounded hope. 



A PLEA FOR LIGHT-WAVES.i 



It is no doubt universally conceded that no era in the world's 

 history has ever seen such immense and rapid strides in the prac- 

 tical applications of science as that in which it is our good fortune 

 to live. Especially true is this of the wonderful achievements in 

 the employment of electricity for almost every imaginable purpose. 

 Hardly a problem suggests itself to the fertile mind of the inven- 

 tor or investigator without suggesting or demanding the applica- 

 tion of electricity to its solution. 



If we except the exquisite results obtained in the manufacture 

 and use of diffraction gratings, and the very important work ac- 

 complished by the bolometer (a purely electrical invention, by the 

 way), it may well be questioned whether, within the last twenty 

 years, there has been a single epoch-making discovery or invention 

 either in theoretical optics or in its applications. 



It is mainly with a view of attempting to interest brother physi- 

 cists and investigators in this to me most beautiful and fascinat- 

 ing of all branches of physical inquiry, that I venture to present a 

 limited number of problems, and I think promising fields for inves- 

 tigation, in light, together with Some crude and tentative suggestions 

 as to their solution. 



The investigations here proposed all depend upon the phenom- 

 enon of interference of light-waves. In a certain sense all light- 

 prol:)lems may be included in this category, but those to which I 

 wish to draw your attention are specially those in which a series of 

 light-waves has been divided into two pencils which re-unite in 

 such a way as to produce the well-known phenomenon of interfer- 

 ence fringes. 



The apparatus by which this is effected is known by the incon- 

 venient and somewhat inappropriate name of ' interferential re- 

 fractometer.' As the instrument which I had the honor of describ- 

 ing to the section at the last meeting is simple in construction, and 

 has already proved its value in several experiments already completed 

 and in the preliminary work of others now under way, I may be 

 permitted to recall the chief points of its construction and theory.' 

 A beam of light falls on the front surface of a plane parallel piece 

 of optical glass at any angle, — usually forty-five degrees, — part 

 being reflected, and part transmitted. The reflected portion is re- 

 turned by a plane mirror, normal to its path, back through the 

 inclined plate. The second or transmitted portion is also returned 

 by a plane mirror, and is in part reflected by the inclined plate, 



1 Abstract of an address before the Section of Physics of the American Association 

 for the Advancement of Science, at Cleveland, O., Aug. 15-22, 1888, by Albert A. 

 Michelson, vice-president of the 



