TRANSACTIONS OF THE SECTIONS. 39 



Thu3 a vacuum is obtained sufficient to allow tlie passage of the current, and at 

 the same time the gas is constantly renewed, which prevents its decomposition. 

 The spectrum of ammonia consists of a broad gi-eenish-yellow band, the wave-length 

 of which was determined by interpolation to be 5G86 to 5G27 10th metres. 



Having no Sprengel's pump at my disposal, I could not examine hydrogen under 

 the conditions in which it is said to give a third spectrum : suffice it to say that 

 Pliicker haSoCxamined it under those circumstances, and does not mention any new 

 spectrum. Angstrom has shown that all the lines of this spectrum coincide with 

 lines of sidphur (which might be introduced by the caoutchouc tubings). Wiillner 

 says that the general appearance of the sulphur-spectrum is a different one ; but 

 this may be due to the cn-cumstance that the sulphur-spectrum was never examined 

 under so minute a pressure, 



On tJie Application of Photocjraphy to copy Diffraction-gratings. 

 By the Hon. J. "W. Steutt. 



Great interest has always attached itself to the beautiful phenomena discovered 

 by Frauenhofer, which present themselves when a beam of light falls on a surface 

 rided with a great number of parallel and equidistant lines. Their unexpected 

 character, the brilliant show of colour, and the ready explanation of the main points 

 on the principles of the Wave-theory recommend them to all, while the working 

 phj'sicist recognizes in them the key ^to the exact measurement of wave-lengths, 

 •which has been so splendidly used by Angstrom and others. 



The production, however, of gratings of sufficient fineness and regularity is a 

 matter of no ordinary difficulty. Indeed the exactness required and obtained is 

 almost incredible. The wave-lengths of the two sodium-lines differ by about the 

 thousandth part. If in two gratings, or two parts of the same grating, the average 

 interval between the divisions differed by the fraction, the less refrangible sodium- 

 line of one would be superposed on the more refrangible corresponding to the other. 

 In point of fact the gratings ruled by Nobert of Barth, to whom the scientific 

 world has been greatly indebted, are capable of distinguishing a difference of wave- 

 length probably of a tenth part of that above mentioned. But in order that the 

 U-lines may be resolved at all, there must be no average error (running over a 

 large part of the grating) of -[ oVr P^i"* of the inten'al between consecutive lines. 

 When it is remembered what the interval is (from ^-jVo to o-oWof f^i inch, or even 

 less), the degree of success which has been reached seems very remarkable. 



A work requiring so much accuracy is necessarily costly — the reason, probably, 

 why gratings tit to be used with the telescope for the purpose of showing the fixed 

 lines are comparatively rare. The hope of being able to perfect a process for the 

 reproduction of gratings at a comparatively cheap rate has induced the author to 

 return at the first opportunity to the experiments described in a Preliminary Note 

 read before the Royal Society in June last. Although the subject is as yet by no 

 means exhausted, the author thought it worth while to bring before the Association 

 an account of the progress that has been made, with specimens of the results. 



The method of procedure is very simple. A dry plate prepared by any photo- 

 graphic process on a.Jlat surface of glass or other transparent material not affected by 

 the fluid media employed is brought into contact with the ruled sm'face of the grating 

 in a printing-frame, and exposed to light. In the author's first experiments he used 

 exclusively as a source of light the image of the sun in a lens of short focus placed 

 in the shutter of a darkened room ; but so small a source is not necessary. The light 

 from the clouds or sky reflected hy a mirror through a hole several inches in aperture 

 will be sufficiently concentrated if the frame be a few feet distant. The author has 

 not as yet specially investigated the point, but he believes that if the light be too 

 much diffused, the experiment would tail. Much would, no doubt, depend on the 

 perfection of the contact — an elemeJit verj" likely to vary. The variable intensity 

 of diffused daylight, which it is almost impossible to estimate with precision, has 

 induced him to use exclusively in his later experiments with ordinary photographic 

 plates the light of a moderator lamp. This, with globe removed, is placed at a 

 distance of 1 or 2 feet from the printing-frame, the distance being carefully 



