1872.] Diffraction- gratings by means of Photogrojihy. 415 



Royal Society, reserving the details and some theoretical work connected 

 with the subject for another opportunity. 



It is some years since the idea first occurred to me of taking advantage 

 of the minute delineating power of photography to reproduce with facility 

 the work of so much time and trouble. I thought of constructing a grating 

 on a comparatively large scale, and afterwards reducing by the leus and 

 camera to the required fineness. I am now rather inclined to think that 

 nothing would be gained by this course, that the construction of a grating 

 of a given number of lines and with a given accuracy would not be greatly 

 facilitated by enlarging the scale, and that it is doubtful whether photo- 

 graphic or other lenses are capable of the work that would be required of 

 them. 



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 lines ruled over a square inch, I printed from it on sen- 

 sitive 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. Pvesults of great promise were at once 

 obtained, and after a little practice I found it possible to produce copies 

 comparing not unfavourably with the original. A source of uncertainty 

 lay in the imperfect flatness of the glass on which the sensitive film was 

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

 plate. The remedy is, of course, to use worked glass, which is required 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 differ materially from those of ordi- 

 nary 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 intensifying, 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 examining 

 the solar spectrum, I have not been able to detect any decided 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 collimator. The telescope is made up of a 

 single lens of about thirty inches focus for object-glass, and an ordinary 

 eyepiece held independently. I believe this arrangement to be moie 



