COMMEMORATIVE ADDRESS 9 



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 favor- 

 able physical conditions, and in its installation and use the most careful 

 attention was given to the elimination of errors due to variation of tem- 

 perature, earth tremors, and other disturbances. Not content, how- 

 ever, with perfecting the machinery by which gratings were ruled, Kow- 

 land proceeded to improve the form of the grating itself, making the 

 capital discovery of the concave grating, 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 manufacture 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, 

 Eowland meant that the principal spoils of the victory should be his, 

 and to this end he constructed a diffraction spectrometer of extra- 

 ordinary 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 out- 

 come of this was that wonderful " Photographic Map of the Normal 

 Solar Spectrum," prepared by the use of concave gratings six inches 

 in diameter and twenty-one and a half feet radius, which is recognized 

 as a standard everywhere in the world. As a natural supplement 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 ele- 

 ments, and a large part of this monumental task is already completed, 

 mostly by Rowland's pupils and in his laboratory. 



Time will not allow further expositions of the important conse- 

 quences of his invention of the ruling engine and the concave grating. 



Indeed, the limitations to which I must submit compel the omission 

 of even brief mention of many interesting and valuable investigations 

 relating to other subjects begun and finished during these years of 

 activity in optical research, many of them by Eowland himself and 



