416 On the Reproduction of Diffraction-gratings. [June 20, 



efficient than a common spectroscope, with collimator and telescope all on 

 one stand ; at any rate, the magnifying-power is considerably greater, and 

 it seems to be well borne. 



I have also experimented on the reproduction of gratings by a very 

 different kind of photography. It will be remembered that a mixture of 

 gelatine with bichromate of potash is sensitive to the action of light, becoming 

 insoluble, even in hot water, after exposure. In ordinary carbon printing 

 the colouring- matter is mixed with the gelatine and the print developed 

 with warm water, having been first transferred so as to expose to the action 

 of the water what was during the operation of the light the hind surface. 

 In my experiments the colouring-matter was omitted, and the bichromated 

 gelatine poured on the glass like collodion and then allowed to dry in the 

 dark. A few minutes' exposure to the direct rays of the sun then sufficed 

 to 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 trans- 

 parent, so that the cause of the peculiar effect must lie in an alternate ele- 

 vation 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 gelatine 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 diffi- 

 cult 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 carbon printing. 



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

 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 concen- 

 trated in one spectrum is only about -j 3 ^, and that this happens 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 substituted, 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. 



