140 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



to run short. He succeeds in producing this state of hydro- 

 gen saturation better by employing diluted sulphuric acid 

 as the electrolytical liquid than with distilled water. 7 A, 

 XLVII.,145. 



DIFFRACTION GRATINGS. 



The fine-ruled gratings that give such perfect spectra of 

 the light falling upon them have of late years become very 

 important in all experiments relating to spectrum analysis. 

 Indeed, they promise to supplant entirely the ordinary glass 

 prism in certain classes of work, especially in that relating 

 to the heat spectra. A memoir by Lord Rayleigh gives some 

 interesting accounts of the methods of manufacturing these 

 gratings, and a more elaborate investigation of the theory of 

 their action than is to be found elsewhere. The original grat- 

 ings, as made by Nobert, of Paris, and Rutherford, of New 

 York, are evidently produced by very delicate and exact au- 

 tomatic ruling-machines, by means of which a glass or metal- 

 lic plate may be ruled by a fine diamond point with lines so 

 closely crowded that at least 6000 grooves are contained in 

 one inch ; and the regularity of the rulings is such, as shown 

 by the purity of the spectra, that a systematic irregularity 

 in the width of the grooves to the extent of the one-thou- 

 sandth part of their own intervals, or the six-millionth part 

 of an inch, does not exist. Having obtained specimens of 

 the original gratings, Lord Rayleigh desired to devise some 

 method of reproducing them, and attempted first of all to 

 do so by photography. Having endeavored to photograph 

 coarse gratings upon glass, so as to obtain, in their dimin- 

 ished pictures, fine-ruled gratings, and having found that the 

 inherent imperfections of the optical appliances, if not the 

 laws of light themselves, interpose an almost insuperable ob- 

 stacle to obtaining adequate results, the author then resorted 

 to the method of contact printing, in which the photographic 

 film is brought, by moderate pressure in a printing-frame, 

 within a very short distance of the lines of the original grat- 

 ing. If, then, the source of light be moderately small, and 

 the rays fall perpendicularly, the copy rarely fails in defini- 

 tion, unless there is some photographic defect. In this way 

 he has copied gratings of 3000 and 6000 lines to the linear 

 inch ; he finds that, on actual trial, the spectra given by the 



