Many important contributions to the theory and practice of spectrum analysis have been made since the time of Kirchoff and Bunsen, only two or three of which can be referred to here. Instrumental methods by which spectra are produced and examined have been greatly perfected, and this is especially true of what is known as the “diffraction grating” first used by Fraunhofer. A quarter of a century ago Rutherford, of New York, constructed a ruling engine by means of which gratings on glass and spectrum metal were ruled with a precision greatly exceeding what had before been possible. A few years later Rowland, of Baltimore, made a notable advance in the construction of a screw far more perfect than any before made, producing gratings of a fineness and regularity of spacing far ahead of any others, and especially by the capital discovery of the concave grating, by means of which the most beautiful results have been obtained. Very recently Michelson, of Chicago, has invented the echelon spectroscope, which, although greatly restricted in range, exceeds all others in power of analysis of spectral lines. In his hands this instrument has been most effective in the study of the influence of a strong magnetic field upon the character of the spectrum from light produced therein, a most interesting phenomenon first observed by Zeeman and one which promises to reveal much concerning the relation of molecular activity to light and to magnetic force. The development of spectrum analysis was necessarily accompanied by a recognition of the identity of radiant heat and light. The study of radiant heat, which was carried on during the earlier years of the century by Leslie, and later by Melloni and Tyndall, by what might be called thermal methods, has been industriously pursued during the last two decades by processes similar to those adopted for visual radiation. The most notable contribution to this work is the invention of the _bolometer_, by Langley, who, at Allegheny, and later at Washington, has made exhaustive studies of solar radiation in invisible regions of the spectrum, especially among the waves of greater length than those of red light, where he has found absorption lines and bands similar in character to those observed in the visible spectrum. He has also studied the absorption of the earth’s atmosphere, the relation of energy to visual effect, and many other interesting problems, the solution of which was made possible by the use of the bolometer. Mention must also be made of the invention by Michelson of an interference comparator, by means of which linear measurements by optical methods can be accomplished with a degree of accuracy hitherto unheard of. With this instrument Michelson has determined the length of the international prototype metre in terms of the wave length of the light of a particular spectral line, thus furnishing for the first time a satisfactory _natural_ unit of length.