August 6, 1896J 



NA TURE 



333 



in which the first half was ruleil at the rate of looo to the inch, 

 ami the second half at the rate of looi to the inch, the one half 

 would evidently do the same thing for one soda line as the other 

 half of the grating was doing for the other soda line, and the two 

 lines would he mixed together and confused. In order, there- 

 fore, to do anything like good work, it is necessary, not only .to 

 have a very great number of lines, hut to have Ihem spaced with 

 most extraonlinary precision; and it is wonderful what success 

 has been reached by the beautiful dividing machines of Ruther- 

 furd and Rowland. I have seen Rowland's machine at Baltimore, 

 and have heard him speak of the great precautions required to 

 get good results. The whole operation of the machine is auto- 

 matic ; the ruling goes on continuously day and night, and it is 

 necessary to pay the most careful regard to uniformity of 

 temperature, for the slightest expansion or contraction due to 

 change of temperature of the different parts of the machine 

 would bring utter confusion into the grating and its resulting 

 spectrum. 



In printing, the contact has to be pretty close, and the finer 

 the grating the closer must the contact be. I experimented 

 upon that point by preparing a photographic film upon a slightly 

 convex surface, and using that for the print ; then, where the 

 contact was closest, the original of course was very well impressed, 

 and round that, one got different degrees of increasingly 

 imperfect contact, and one could trace in the result what the 

 eflfect of imperfect contact is. I found that, both with gratings 

 of 3000 and 6000 lines to the inch, good enough contact was 

 obtained with ordinary flat glass ; but when youcome to gratings 

 of 17,000 or 20,000 lines to the inch the contact requires to be 

 extremely close, and in order to get a good copy of a grating 

 with 20,000 lines per inch it is necessary that there should 

 nowhere be one ten-thousandth of an inch between the original 

 and the printing surface — a degree of closeness not easily secured 

 over the entire area. It is rather singular that though I published 

 full accounts of this work a long time ago, and distributed a 

 large number of copies, the process of reproducing gratings by 

 photography did not become universally known, and was 

 re-discovered in France, by Isarn, only two or three years 

 since. 



One reason why photographic reproduction is not practised to 

 a very great extent is, that the modern gratings— such as 

 Rowland's — are ruled almost universally upon speculum metal. 

 A grating upon speculum metal is very excellent for use, but does 

 not well lend itself to the process of photographic copying, 

 although I have succeeded to a certain extent in copyiiig 

 a grating ruled upon speculum metal. For this purpose 

 the light had to pass first through the photographic film, 

 then be reflected from the speculum metal, and so pass back 

 again through the film. (Iratings .such as could easily be 

 made by copying from a glass original are not readily produced 

 from one on speculum metal, and I think that is the reason why 

 the process has not come into more regular use. Glass is much 

 more trying than speculum metal to the diamond, and that 

 accounts for the latter being generally ]ireferred for gratings ; 

 indeed, the principal difficulty consists in getting a good diamond 

 point, and maintaining it in a shape suitable for making the very 

 fine cut which is required. 



I may now allude to another method of photographic repro- 

 duction which I tried only last summer. It happened that I 

 then went with Prof. Meldola over VVaterlow's large photo- 

 mechanical printing establishment, and I was very much 

 interested, among many other very interesting things, in the use 

 of the old bitumen process — the first photographic process 

 known. It is used for the reproduction of cuts in black and 

 white. A carefully cleansed zinc plate is coated with varnish of 

 bitumen dissolved in benzole, and exposed to sunlight for about 

 two hours under a negative, giving great contrast. Where the 

 light penetrates the negative the bitumen becomes comparatively 

 insoluble, and where it has been protected from the action of 

 light it retains its original degree of solubility. When the ex- 

 posed plate is treated with a solvent, tur|)entine or some solvent 

 milder than benzole, the protected parts are dissolved away, 

 leaving the bare metal ; whereas the parts that have received 

 the sunlight, being rendered insoluble, remain upon the metal 

 and protect it in the sub.sequent etching process. I did not 

 propose to etch metal, and, therefore, I simply used the bitumen 

 varnish spread upon glass plates, and exposed the plates so pre- 

 pared to sunshine for about two hours in contact with the grating. 

 They are then developed, if one may use the phrase, with turpen- 

 tine ; and this is the part of the process which is the most difficult 



NO. 1397, VOL. 54] 



to manage. If you stop development early you get a grating 

 which gives fair spectra, but it may be deficient in intensity and 

 brightness; if you push development, the brightness increases up 

 to a point at which the film disintegrates altogether. In this way 

 one is tempted to pursue the process to the very last point, and, 

 although one may succeed so far as to have a film which is quite 

 intact so long as the turpentine is upon it, I have not succeeded 

 in finding any method of getting rid of the turpentine without 

 leading to the disintegration of the film. In the commercial 

 application of the process the bitumen is treated somewhat 

 brutally — the turpentine is rinsed oft' with a jet of water ; I have 

 tried that, and many of my results have been very good. I have 

 also tried to sling off the turpentine by putting the plate into a 

 kind of centrifugal machine ; but by either plan the film in which 

 the development has been too far pushed, is liable not to survive 

 the treatment required for getting rid of the turpentine. If the 

 solvent is allowed to remain we are in another difficulty, because 

 then the developing action is continued and the result is lost. But 

 if the process is properly managed, and development stopped at 

 the right point, and if the film be of the right degree of thick- 

 ness, you get an. excellent copy. I have one here, 6000 

 lines to the inch, which I think is about the very best copy I 

 have ever made. The method gives results somewhat superior 

 to the best that can be got with gelatine ; but I would not recom- 

 mend it in preference to the latter, because it is very much more 

 difficult to work unless some one can hit upon an improved 

 manipulation. 



I will not enlarge upon the importance of gratings ; those 

 acquainted with optics know how very important is the part 

 played by diffraction gratings in optical research, and how the 

 inost delicate w-ork upon spectra, requiring the highest degree 

 of optical power, is made by means of gratings, ruled on 

 speculum metal by Rowland. I suppose the reason why no 

 professional photographer has taken up the production of photo- 

 graphic gratings, is the difficulty of getting the glass originals ; 

 they are very expensive, and I do not know where they are now 

 to be obtained. It seems a pity that photographic copies should 

 not be more generally available. I have given a great many 

 away myself ; but educational establishments are increasing all 

 over the country, and for the purpose of instructing students it 

 is desirable that reasonably good gratings should be placed in 

 their hands, to make them familiar with the measurements by 

 which the wave-length of light is determined. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Mr. Stanley Dunkeri.ey has been appointed to the Uni- 

 versity Demonstratorship of Mechanism and Applied Mechanics 

 at Cambridge, made vacant by the election of Mr. Dalby to the 

 Professorship of Mechanical Engineering at Finsbury College. 



Among the recipients of honorary degrees, conferred at the 

 close of the summer session of the University of Edinburgh on 

 Saturday, were Prof. Francis .\. Walker. President of the Massa- 

 chusetts Institute of Technology, and Sir Dietrich Brandis, 

 K.C.I.E., F.R.S., late Inspector-General of Forests in India. 



Dr. J. D. Porter, of Columbia College, New York, has 

 been appointed to the newly-founded Macdonald chair of 

 Mining and Metallurgy in the M'Gill University, Montreal. 

 Mr. Herbert W. Umney, of Bath, has been appointed Assistant- 

 Professor of Civil Engineering. 



The Council of the Hartley Institution, Southampton, have 

 just made the following appointments ;— Lecturer in Mathe- 

 matics, Dr. Cuthbert E. Cullis, Assistant Lecturer to Prof. 

 Karl Pearson, University College, London. Lecturer in 

 Chemistiy, Dr. D. R. Boyd, Demonstrator and Assistant 

 Lecturer in Chemistry, Mason College, Birmingham. Lecturer 

 in Biology and Geology, Mr. E. T. Mellor, Assistant 

 Demonstrator in Biology, Owens College, Manchester. 



Her Majesty's Commissioners for the E.xhibition of 1S51 

 have made the following appointments to Science Research 

 Scholarships, for the year 1896, on the recommendation of the 

 authorities of the respective Universities and Colleges. The 

 scholarships are of the value of ^150 a year, and are ordinarily 

 tenable for two years (subject to a satisfactory report at the end 

 of the first year) in any University at home and abroad, or in 

 some other institution approved of by the Commissioners. The 



