Reproduction of Diffraction- gratings by means of Photography. 393 



However this may be, the method that I adopted is better in 

 every respect, except perhaps one. Having provided myself with a 

 grating by Nobert, with 3000 hnes ruled over a square inch, I printed 

 from it on sensitive dry plates in the same way as transparencies for 

 the lantern are usually printed from negatives. 



In order to give myself the best chance of success, I took as a 

 source of light the image of the sun formed by a lens placed in the 

 shutter of a dark room. I hoped in this way that, even if there 

 should be a small interval between the lines of the grating and the 

 sensitive surface, still a shadow of the lines would be thrown across 

 it. Results of great promise were at once obtained, and after a little 

 practice I found it possible to produce copies comparing not un- 

 favourably with the original. A source of uncertainty lay in the 

 imperfect flatness of the glass on which the sensitive film was pre- 

 pared, though care was taken to choose the flattest pieces of patent 

 plate. The remedy is, of course, to use worked glass, which is re- 

 quired in any case if the magnifying-power of a telescope is to be 

 made available. 



Almost any of the dry processes known to photographers may be 

 used. I have tried plain albumen, albumen on plain collodion, and 

 Taupenot plates. The requirements of the case diff'er materially from 

 those of ordinary photography, sensitiveness being no object, and 

 hardness rather than softness desirable in the results. After partial 

 development, I have found a treatment with iodine, in order to clear 

 the transparent parts, very useful. In proceeding with the intensi- 

 fying, the deposit falls wholly on the parts that are to be opaque. 

 It is more essential that the transparent parts should be quite clear 

 than the dark parts should be very opaque. 



The performance of these gratings is very satisfactory. In ex- 

 amining the solar spectrum, I have not been able to detect any de- 

 cided inferiority in the defining-power of the copies. With them, 

 as with the original, the nickel line between the D's is easily seen in 

 the third spectrum. I work in a dark room, setting up the grating 

 at a distance from the slit fastened in the shutter, and using no col- 

 limator. The telescope is made up of a single lens of about thirty 

 inches focus for object-glass, and an ordinary eyepiece held inde- 

 pendently. I believe this arrangement to be more eflicient 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 difl^erent 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 ge- 

 latine 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 sufliced to 



