746 COMMEMORATION OF PROE\ HENRY A. ROWLAND. 



and would be good or bad just as these were few or many. The prob- 

 lem was, then, to make a screw which would be practically free from 

 periodic and other errors, and upon this problem a vast amount of 

 thought and experiment had already been expended. Rowland's solu- 

 tion of it was characteristic of his genius; there were no easy advances 

 through a series of experiments in which success and failure mingled in 

 varying proportions; "fire and fall back" was an order which he 

 neither gave nor obeyed, capture by storm being more to his mind. 

 He was by nature a mechanician of the highest type, and he was not 

 long in devising a method for removing the irregularities of a screw, 

 which astonished everybody by its simplicity and by the all but abso- 

 lute perfection of its results. Indeed, the very first screw made by 

 this process ranks to-day as the most perfect in the world. But such 

 an engine as this might only be worked up to its highest efficiency 

 under the most favorable physical conditions, and in its installation 

 and use the most careful attention was given to the elimination of 

 errors due to variation of temperature, earth tremors, and other dis- 

 turbances. Not content, however, with perfecting the machinery by 

 which gratings were ruled, Rowland proceeded to improve the form 

 of the grating itself, making the capital discovery of the concave grat- 

 ing, by means of which a large part of the complex and otherwise 

 troublesome optical accessories to the diffraction spectroscope might 

 be dispensed with. Calling to his aid the wonderful skill of Brashear 

 in making and polishing plane and concave surfaces, as well as the 

 ingenuity and patience of Schneider, for so many years his intelligent 

 and loyal assistant at the lathe and workbench, he began the manufac- 

 ture and distribution, all too slowly for the anxious demands of the 

 scientific world, of those beautifully simple instruments of precision 

 which have contributed so much to the advance of physical science 

 during the past twenty years. While willing and anxious to give the 

 widest possible distribution to these gratings, thus giving everywhere 

 a new impetus to optical research, Rowland meant that the principal 

 spoils of the victory should be his, and to this end he constructed a 

 diffraction spectrometer of extraordinary dimensions and began his 

 classical researches on the solar spectrum. Finding photography to 

 be the best means of reproducing the delicate spectral lines shown by 

 the concave grating, he became at once an ardent student and, shortly, 

 a master of that art. The outcome of this was that wonderful " Pho- 

 tograhic Map of the Normal Solar Spectrum," prepared by the use of 

 concave gratings 6 inches in diameter and 21£ feet radius, which is 

 recognized as a standard everywhere in the world. As a natural sup- 

 plement to this he directed an elaborate investigation of absolute wave- 

 lengths, undertaking to give, finally, the wave-length of not only every 

 line of the solar spectrum, but also of the bright lines of the principal 

 elements, and a large part of this monumental task is already com- 

 pleted, mostly by Rowland's pupils and in his laboratory. 



