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ADDRESS BY ALBERT A. MICHELSON.
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
which there is a considerable difference in path between the two pencils.
The instrument which $ had the honor of describing to the section at the last meeting is free from all the objections mentioned. It is simple in construction ; with a little familiarity it is easily adjusted ; it may be used with a broad luminous surface ; the pencils may be separated as far as desired ; and when properly adjusted the position of the fringes is perfectly definite.1 As an additional advantage it may be stated that this is probably the onty form of instrument which permits the use of white light (and therefore of the identification of the fringes) without risk of disturbing the position of either surface by contact or close approximation. It is chiefly this property which renders this instrument peculiarly adapted to the comparison of standards of length.
As this form of refractometer 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 returned b}' a plane mirror, normal to its path, back through the inclined plate. The second or transmitted portion is tilso returned by a plane mirror and is in part reflected by the incited plate,/tlms coinciding with the transmitted part of the first pencil; and the two pencils are thus brought to “ interfere.”2 A little consideration will show that this arrangement is exactty equivalent to an air-film or plate between two plane surfaces. The interference phenomena are therefore the same as for such an air-plate.
If the virtual distance between the plane surfaces is small, white light ma}^ be employed and we have then colored fringes like Newton’s rings or the colors of a soap-film. If the distance exceeds a few wave lengths, monochromatic light must be employed.
1 It may Incidentally be mentioned that extraneous reflections—such as usually accompany most ol' the phenomena of interference—may be almost entirely avoided by ft transparent Him of silver on the front snrfacc of the glass plate where the rays separate; and accordingly the fringes in white light present a purity and gorgeousness of coloration that is only rivaled by the colors of the pulariscope.
* A second plane parallel plate of the same thickness and Inclination is placed (for compensation) in the path of the first pencil.