326 PROGRESS IN PHYSICS. 



years later Rowland, of Baltimore, made a notable advance in the con- 

 struction of a screw fai'more perfect than any before made, producing 

 gratings of a iineness and regularity of .spacing far ahead of any 

 others, and especially by a 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 spec- 

 troscope, which, although greatly restricted in range, exceeds all 

 others in power of analysis of spectral lines. In his hands this instru- 

 ment 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, 

 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 stud}^ 

 of radiant heat, which was carried on during the earlier 3^ears of the 

 century by Leslie, and later Ijv Melloni and Tyndall, by what might 

 be called thermal methods, has been industi'iously pursued during the 

 last two decades by processes similar to those adopted for visual radi- 

 ation. The most notable contribution to this work is the invention of 

 the bolometer by Langley, who, at Allegheny and later at Washing- 

 ton, 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. whei"e he has found absorption lines and bands 

 simOar in character to those observed in the visible spectrum. He 

 has also studied the absorption of the earth's atmosphere, the relation 

 of energ}^ 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 b}^ Michelson of an 

 interference comparator, by means of which linear measurements by 

 optical methods can be accomplished with a degree of accuracy hith- 

 erto unheard of. With this instrument Michelson has determined the 

 length of the international prototj^pe meter in terms of the wave 

 length of the light of a particular spectral line, thus furnishing for 

 the first time a satisfactor}^ natural unit of length. 



By far the most important contribution to the theory of light made 

 during the last half of the century is that of Maxwell, who, in 1873, 

 announced the proposition that electro-magnetic phenomena and light 

 phenomena have their origin in the same medium and that they are 

 identical in nature. This far-reaching conclusion has been generally 

 accepted and forms the basis of much of the most important work in 

 physical research in process of elal)oration as the century closes. To 

 some of this reference will presently be made. 



