29 



PRELIMINARY NOTICE OF THE RESULTS ACCOMPLISHED 

 IN THE MANUFACTURE AND THEORY OF GRATINGS FOR 

 OPTICAL PURPOSES 



[Johns Hopkins University Circulars, No. 17, pp. 248, 249, 1882 ; Philosophical Magazine 

 [4], XIII, 469-474, 1882; Nature, 26, 211-213, 1882; Journal de Physique, 

 II, 5-11, 1883] 



It is not many years since physicists considered that a spectroscope 

 constructed of a large number of prisms was the best and only instru- 

 ment for viewing the spectrum, where great power was required. These 

 instruments were large and expensive, so that few physicists could pos- 

 sess them. Professor Young was the first to discover that some of the 

 gratings of Mr. Rutherfurd showed more than any prism spectroscope 

 which had then been constructed. But all the gratings which had been 

 made up to that time were quite small, say one inch square, whereas 

 the power of a grating in resolving the lines of the spectrum increases 

 with the size. Mr. Rutherfurd then attempted to make as large grat- 

 ings as his machine would allow, and produced some which were nearly 

 two inches square, though he was rarely successful above an inch and 

 three-quarters, having about thirty thousand lines. These gratings 

 were on speculum metal and showed more of the spectrum than had 

 ever before been seen, and have, in the hands of Young, Rutherfurd, 

 Lockyer and others, done much good work for science. Many mechanics 

 in this country and in France and Germany, have sought to equal 

 Mr. Rutherfurd' s gratings, but without success. 



Under these circumstances, I have taken up the subject with the 

 resources at command in the physical laboratory of the Johns Hopkins 

 University. 



One of the problems to be solved in making a machine is to make a 

 perfect screw, and this, mechanics of all countries have sought to do 

 for over a hundred years and have failed. On thinking over the matter, 

 I devised a plan whose details I shall soon publish, by which I hope to 

 make a practically perfect screw, and so important did the problem seem 

 that I immediately set Mr. Schneider, the instrument maker of the 

 university, at work at one. The operation seemed so successful that I 



