416 LEADING AMERICAN MEN OF SCIENCE 



satisfactory gratings could be made, and he set himself to work at 

 this problem. In order to get good gratings a screw as nearly 

 perfect as possible must be made. But to make such a screw is 

 an extremely difficult matter. By a simple and ingenious device 

 the difficulties were largely overcome and, with the aid of the 

 historic screw which resulted, gratings far superior to any that 

 had previously been known were made. By the motion of the 

 screw the plate which rested upon it was moved slowly and regu- 

 larly forward while a diamond point moved across its surface. 

 The first gratings made were plane, but soon it occurred to the 

 inventor that, if the lines should be ruled upon a concave surface, 

 it would be possible to photograph spectra directly without the 

 use of prisms and lenses and with much better results in every 

 way. I happened to be with Rowland when the idea of the con- 

 cave grating occurred to him. We were on our way from Balti- 

 more to Washington to attend a meeting of the National Academy 

 of Sciences. He was very quiet and we sat together almost the 

 whole way without a word passing between us. This, however, 

 was not unusual. We talked when we wanted to and, as often 

 happened, we didn't want to. Well, in this instance I think I 

 noticed that my friend was brooding more intently than usual. 

 Just before we reached Washington he threw up his hands and 

 said, "It will work. I'm sure of it." These words were not 

 addressed to me but to space. I naturally wanted to know what 

 he was talking about, but at first he could not bring himself to 

 explain. Presently, however, he told me that he intended to go 

 back at once to Baltimore to make preparations for ruling 

 gratings on concave plates. He was positive he could do it and 

 he saw at once the great advantages of this form of apparatus. 

 My recollection is that he gave up the meeting of the National 

 Academy and returned on the next train to Baltimore. At all 

 events, it was not long before the first concave grating was ready 

 for use and Rowland's maps of the solar spectrum in course of 

 preparation. In a notice of his work recently written by a leading 

 English physicist reference is made to the maps in these words: 

 "The beautiful maps issued at a later date by Rowland, . . . 



