Albert A. Michelson, "A Plea for Light Waves", Proceedings, AAAS, Section B, 37, 1888.

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voted almost their whole lives to the pursuit of this most fascinating study.

In the enthusiasm aroused by so many wonderful, beautiful and bewildering results, such varfed and far-reaching discoveries in the vast fields of this subtile and powerful agent, it is not to be wondered at—or indeed entirely to be regretted—that possibly a great deal of attention has been diverted from the sister department of light. Undoubtedly, there have been man v important developments and improvements in optical instruments — the microscope, the telescope, the spectroscope and the camera may be said to lAve reached the point of practical perfection — and it is equally true that the observations and discoveries made with the help of these have more than justified the high expectations which their advent created. Certainly the wonderful impulse the study of biology has received by the revelations of the microscope; the enormous increase in our knowledge of the size, distance and motions of llie heavenly bodies, due chiefly to the little less than marvellous power and precision of our telescopes ; the knowledge of solar and stellar physics — which a few years ago would have been thought visionary if not impossible — attained by#the spectroscope, now so happily supplemented by the camera; the insight gained into the structure of matter by spectroscopic interpretation of the messages which its molecules impart to the luminifeuous ether,—these are all even more truly wonderful and important than any;of the astonishing marvels of electricity. But their original conception belongs to an era that is past*

If we except the exquisite results obtained in the manufacture and use of diffraction gratings and the very important work accomplished 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 inventi6n either in theoretical optics or in its applications.

It may, perhaps, be argued that the department of optics has been fairly completed ; that its theory (though still imperfect in many important points) has been fairly well developed and the range of its applications fairly well understood. Tlijg unexpected wonders it has already accomplished make it somewhat hazardous to reply that these same observations may noiv be applied to electricity and magnetism. Still it is safe to say that at any rate the more important facts and laws as well as the more promising lines



of the development of their applications, are now fairly well known, and that inducements to their further study and development are not wanting.

If, therefore, physicists would devote a larger share of their careful study to the completion of optical theories and to the application of light as an instrument of measurement and investigation— it need never be feared that there 'Would be a lack of electricians to carry forward to their completion, upon lines already well developed, the principles and facts already known.

It is mainly with a view of attempting to interest brother physicists and investigators in this to me most beautiful and fascinating of all branches of physical inquiry that I venture to present a limited number of problems and I think promising fields for investigation in light, together with some crude and tentative suggestions as to their solution.

The investigations here proposed all depend upon the phenomenon of interference of light waves. In a certain sense all light problems 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 reunite in such a way as to produce the well-known phenomenon of interference fringes.

The apparatus by which this is effected is known by the inconvenient and somewhat inappropriate name of “ interferential re-fractometer.” Among the many forms of the apparatus several are fairly well adapted to the work they have already accomplished, but all are open to serious objections. In all the forms which employ abroad luminous source of light, the plane in which the interference fringes are most distinct, is found to vary rapidly with slight changes in adjustment; in fact, it may happen that different portions of the same fringe appear at enormously differentdis-tances, so that it is impossible to fix the true position of a fringe or even to count the number which pass a given fixed point. This very serious objection is avoided by using, as the source of light, a narrow illuminated slit, but of course at a sacrifice of light and of convenience and simplicity. Both classes of instruments are open to the objection that the two pencils are very close together, rarely more than a centimeter apart. For some purposes this may be an advantage, but for many purposes it is a serious defect. Finally, none of the forms in general use are adapted to experiments in

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