870 



SCIENCE. 



[N. S. Vol. XIV. No. 362. 



Bciences by the aid of a fund originally 

 established by Count Rumford, and in 1881 

 it was crowned as a prize essay by the Vene- 

 tian Institute. Its conclusions have stood 

 the test of twenty years of comparison and 

 criticism. 



In the meantime, Rowland's interest had 

 been drawn, largely, perhaps, through his 

 association with his then colleague, Pro- 

 fessor Hastings, towards the study of light. 

 He was an early and able exponent of Max- 

 well's magnetic theory, and he published 

 important theoretical discussions of electro- 

 magnetic action. Recogniziug the para- 

 mount importance of the spectrum as a key 

 to the solution of problems in ether physics, 

 he set about improving the methods by 

 which it was produced and studied, and 

 was thus led into what will probably always 

 be regarded as his highest scientific achieve- 

 ment. 



At that time the almost universally pre- 

 vailing 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 spec- 

 trum as produced by a grating of fine wires 

 or a close ruling of fine lines on a plane re- 

 flecting or transparent surface had been 

 known for nearly a hundred years, and the 

 colors produced by scratches on polished 

 surfaces were noted by Robert Boyle, more 

 than two hundred years ago. Thomas 

 Young had correctly explained the phe- 

 nomenon 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. Im- 

 perfect as these gratings were, Fraunhofer 

 succeeded in making with them some re- 

 markably good measures of the length of 

 light-waves, and it was everywhere admitted 

 that for the most precise spectrum measure- 

 ments they were indispensable. In their 



construction, however, there were certain 

 mechanical difiiculties 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 many 

 as a hundred thousand in the space of a 

 single inch. The real difficulty was in the 

 lack of uniformity of spacing, and on uni- 

 formity depended the perfection and purity 

 of the spectrum produced. 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 City, 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 some- 

 what complicated set of levers, for which a 

 carefully made screw was afterwards sub- 

 stituted. Aided by the skill and patience 

 of his mechanician, Chapman, Rutherfurd 

 continued 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 satisfactory 

 manner. When Rowland took up the 

 problem, he saw, as, indeed, others had 

 seen before him, that the dominating ele- 

 ment 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 irregularities of this screw, 

 and would be good or bad just as these were 

 few or many. The problem was, then, to 

 make a screw which would be practically 



