394 Royal Society. 



produce such a modification under the lines of the gratings that, 

 on treatment with warm water, a copy of the original was produced 

 capable of giving brilliant spectra. In these gelatine-gratings all parts 

 are alike transparent, so that the cause of the peculiar effect must lie 

 in an alternate elevation and depression of the surface. That this is 

 the case may be proved by pressing soft sealing-wax on the grating, 

 when an impression appears on the wax, giving it an effect like that 

 of mother of pearl. It is known that the effect of water on a gela- 

 tine print is to make the protected parts project in consequence of 

 their greater absorption, but it might have been expected that on 

 drying the whole would have come flat again. It is difficult to say 

 exactly what does happen ; and I am not even sure whether the part 

 protected by the scratch on the original is raised or sunk. Gelatine 

 can scarcely be actually dissolved away, because the uppermost layer 

 must have become insoluble under the influence of the light. I do 

 not at present see my way to working by transfer, as in ordinary car- 

 bon printing. 



I have not yet been able to reduce the production of these gela- 

 tine-gratings to a certainty, but can hardly doubt the possibility of 

 doing so. One or two of considerable perfection have been made, 

 capable of showing the nickel line between the D's, and giving spectra 

 of greater brightness than the common photographs. Not only so, 

 but the gelatine copy surpasses even the original in respect of 

 brightness. The reason is that, on account of the broadening of the 

 shadow of the scratch, a more favourable ratio is established between 

 the breadths of the alternate parts. 



Theory shows that with gratings composed of alternate transparent 

 and opaque parts the utmost fraction of the original light that can 

 be concentrated in one spectrum is only about -^, and that this hap- 

 pens in the first spectrum when the dark and bright parts are equal. 

 But if instead of an opaque bar stopping the light, a transparent bar 

 capable of retarding the light by half an undulation can be substi- 

 tuted, there would be a fourfold increase in the light of the first 

 spectrum. I accordingly anticipate that the gelatine-gratings are 

 likely to prove ultimately the best, if the conditions of their production 

 can be sufficiently mastered. 



With regard to the application of the photographs, I need not say 

 much at present ; it is evident that the use of gratings would become 

 more general if the cost were reduced in the proportion, say, of 20 to 

 1, more particularly if there were no accompanying inferiority of per- 

 formance. 



The specimens sent with this paper are both capable of showing 

 the nickel line and give fairly bright spectra, but they must not be 

 supposed to be the hmit of what is possible. From their appearance 

 under the microscope I see no reason to doubt that lines 6000 to the 

 inch can be copied by the same method, a point which I hope shortly 

 to put to the test of experiment. 



