COMMEMORATION OF PROF. HENRY A. ROWLAND. 745 



ether physics, he set about improving- the methods l)y which it was 

 produced and studied, and was thus led into what will probably always 

 be regarded as his highest scientific achievement. 



At that time the almost universally prevailing method of studying 

 the spectrum was by means of a prism or a train of prisms. But the 

 prismatic spectrum is abnormal, depending for its character largely 

 upon the material made use of. The normal spectrum as produced by a 

 grating of fine wires or a close ruling of fine lines on a plane reflecting 

 or transparent surface had been known for nearly a hundred years, 

 and the colors produced bv scratches on polished surfaces were noted 

 by Robert Bo^de more than two hundred years ago. Thomas Young 

 had correctly explained the phenomenon according to the undulatory 

 theory of light, and gratings of fine wire and, later, of rulings on 

 glass were used by Fraunhofer, who made the first great study of the 

 dark lines of the solar spectrum. Imperfect as these gratings were, 

 Fraunhofer succeeded in making with them some remarkably good 

 measures of the length of lightwaves, and it was ever^'where admitted 

 that for the most precise spectrum measurements they were indispen- 

 sable. In their construction, however, there were certain mechanical 

 difficulties which seemed for a time to be insuperable. There was no 

 special trouble in ruling lines as close together as need be; indeed, 

 Nobert, who was long the most successful maker of ruled gratings, had 

 succeeded in putting as man}^ as 100,000 in the space of a single inch. 

 The real difficulty was in the lack of uniformity of spacing, and on 

 uniformity depended the perfection and purity of the spectrum pro- 

 duced. Nobert jealously guarded his machine and method of ruling 

 gratings as a trade secret, a precaution hardly worth taking, for before 

 many years the best gratings in the world were made in the United 

 States. 



More than thirty years ago an amateur astronomer in New York Citj'^, 

 a lawyer by profession, Lewis M. Rutherfurd, became interested in 

 the subject and built a ruling engine of his own design. In this 

 machine the motion of the plate on which the lines were ruled was 

 produced at first by a somewhat complicated set of levers, for which a 

 carefulh' made screw was afterwards substituted. Aided Ijy the 

 skill and patience of his mechanician, Chapman, Rutherfurd con- 

 tinued to improve the construction of his machine until he was able to 

 produce gratings on glass and on speculum metal far superior to any 

 made in Europe. The best of them, however, were still faulty in 

 respect to uniformity of spacing, and it was impossible to cover a 

 space exceeding two or three square inches in a satisfactor}' manner. 

 When Rowland took up the problem, he saw, as, indeed, others had 

 seen before him, that the dominating element of a ruling machine was 

 the screw by means of which the plate or cutting tool was moved along. 

 The ruled grating would repeat all of the irregularites of this screw 



