SKETCH OF HENRY AUGUSTUS ROWLAND. 117 



ture of the water changed, the slight expansion caused by the 

 friction would have made the threads vary slightly. This would 

 have caused the carriage that runs on it to vary slightly, and con- 

 sequently the spaces between the grooves on the " grating " would 

 vary, and render it useless for scientific purposes. The screw is 

 turned by a solid steel wheel with seven hundred and fifty teeth 

 on the ring, which is moved the space of one tooth at a time 

 by an ingenious contrivance attached to the driving shaft. The 

 screw having twenty threads, the carriage is moved one fifteen- 

 thousandth part of an inch each time, thus making that many 

 grooves to the inch on the metal " grating." The number of 

 grooves may be regulated. " Gratings " have been made with 

 forty-eight thousand grooves to the inch. By the strongest micro- 

 scope made, the human eye could not see the lines if there were 

 more than a hundred and twenty-five thousand to the inch. Prof. 

 Rowland says he gets the best results from " gratings " with fif- 

 teen thousand grooves to the inch. The machine now in use at 

 Johns Hopkins University is the third of the kind made. The 

 first was completed years ago, and is still in use in the vault. 

 The European, and especially the German universities, have tried 

 repeatedly to make a machine of the kind, but have never suc- 

 ceeded. Hence all their best universities get the "gratings" for 

 their spectroscopes from the machine at the Hopkins. 



When a " grating " is completed, it is taken out, tested, packed 

 in a handsome hardwood case, and sent to Mr. Brashear in Alle- 

 gheny, Pa. This gentleman attends to the sale of the valuable 

 " gratings," which cost from twenty to three hundred dollars. The 

 proceeds are divided between Mr. Brashear and the university. 



When these tiny grooves, cut with a diamond point on the 

 polished metal plate, are completed and are perfect, the grating 

 breaks up a ray of light into its various colors as a prism does. 

 Some of the gratings produce " ghosts," and are then considered 

 imperfect. Prof. Rowland deals with these "ghosts" of the spec- 

 trum in a recent article in the Astro-Physical Journal, of Chi- 

 cago. He says: "A periodic displacement of one millionth of 

 an inch in a grating will produce visible ghosts which are seen 

 in the second spectram and are troublesome in the third. With 

 very bright spectra these might even be seen in the first spec- 

 trum. An over-exposed photographic plate would readily bring 

 them out." 



With the concave grating Prof. Rowland has made an im- 

 mense photographic map of the solar spectrum, and has deter- 

 mined a system of standard wave lengths which is now univer- 

 sally adopted. He is now having measured the wave length of 

 every line of the solar spectrum and is determining the elements 

 to which they all belong. This is a work of years, as is also the 



