THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



FEBRUARY 1874. 



XII. On the Manufacture and Theory of Diffraction- gratings. 

 By Lord Rayleigh, F.R.S* 



IN a " Preliminary Note on the Reproduction of Diffraction- 

 gratings by means of Photography," published in the Pro- 

 ceedings of the Royal Society for June 20, 1872, and in the 

 Philosophical Magazine for November of the same year, I gave 

 a short account of experiments with which I had been for some 

 time occupied. A few further details were communicated to the 

 British Association at Brighton (Brit. Assoc. Report, p. 39). I 

 now propose to give the results of more recent experience in the 

 practical manufacture of gratings, as well as some theoretical 

 conclusions which have been in manuscript since the subject 

 first engaged my attention. 



There are two distinct methods of copying practised by the 

 photographer — (1) by means of the camera, (2) by contact- 

 printing. The first, if it were practicable for our purpose, would 

 have the advantage of leaving the scale arbitrary, so that copies 

 of varying degrees of fineness might be taken from the same 

 original. By this method I have obtained a photograph of a 

 piece of striped stuff on such a scale that there was room for 

 about 200 lines in front of the pupil of the eye, capable of show- 

 ing lateral images of a candle ; but I soon found that the inhe- 

 rent imperfections of our optical appliances, if not the laws of 

 light themselves, interposed an almost insuperable obstacle to 

 obtaining adequate results. 



However perfect a lens may be, there is a limit to its powers 

 of condensing light into a point. Even if the source from which 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 47. No. 310. Feb. 1874. G 



