L98 Lord Rayleigh on Copying JJij/'ractlon-g rating s f 



In my former paper 1 stated my opinion that the photo- 

 graphic method of reproduction would be applicable to lines 

 liner than any that I had then tried (G000 to inch). In the 

 summer of 1879 an opportunity afforded itself of submitting 

 the matter to the test of actual trial through the kindness of 

 Mr. Rutherfurdj who presented me with a beautiful glass 

 grating containing nearly 12,000 lines, ruled at the rate of 

 17,280 to the inch. The copies, taken with suitable pre- 

 cautions to secure a good contact, were completely successful, 

 so far as the spectrum of the first order is concerned. Indeed 

 careful comparison showed no appreciable difference between 

 the defming-power of the original and of the copies ; and with 

 respect to brightness some of the copies had the advantage. 

 On a former occasion * I have shown that the theoretical 

 resolving-power in the orange region of the spectrum is 

 equal to that obtainable from a prismatic spectroscope with 

 12i per cent, of " extra dense flint ;" and I have no reason to 

 think that the actual resolving-power fell far short. This is a 

 considerable result to obtain with a photograph which may 

 be taken in half an hour at a cost of two or three shillings. 



The case is different, however, when we turn to the spectrum 

 of the second order. Used in this way the original gives 

 magnificent results ; but they are not reproduced in the copies. 

 Some parts of the photograph will sometimes show a faint 

 spectrum of the second order ; but it is usually traversed by 

 one or more dark bands, whose nature will presently be ex- 

 amined more at length, 



As a rule, glass (or at any rate transparent) originals only 

 would be used for purposes of reproduction ; but as a matter 

 of curiosity I tried what could be done in copying an original 

 ruled on speculum-metal. The specimen experimented upon 

 was similar to my own, both as to the total number of lines 

 and as to the degree of closeness, and belongs to Mr. Spottis- 

 woode, to whom I am indebted for the loan of it. In this 

 case the light of the sun had to pass through the sensitive 

 film before it could reach the speculum-metal ; it was then 

 reflected back, and in returning through the film impressed 

 the ruled structure. No very brilliant result was to be ex- 

 pected ; but I succeeded so far as to obtain a copy which gave 

 very fair results when tested upon the sun. 



In my former paper I mentioned that when a spectrum of 

 high order is thrown upon the eye, there usually appear upon 

 the grating a certain number of irregular dark bands. These 

 are the places at which the copy fails to produce the spec- 

 trum in question. With lines not closer than 3000 or 6000 

 * Phil. Mag. Oct, 1879. 



