NA TURE 



[August 6, 1896 



THE REPRODUCTION OF DIFFRACTION 

 GRA TINGS} 



I HAVE first to apologise for the very informal character of 

 the communication which I am about to make to the club ; 

 I have not been able to put anything down upon paper, but I 

 thought it might be interesting to some to hear an account of 

 experiments that have now been carried on at intervals for a 

 considerable series of years in the reproduction — mainly the 

 photographic reproduction — of diflVaction gratings. Probably 

 most of you know- that these consist of straight lines ruled very 

 closely, very accurately, and parallel to one another, upon a 

 piece of glass or speculum metal. Usually they arc ruled with 

 a diamond by the aid of a dividing machine ; and in late years, 

 particularly in the hands of Rutherfurd and Rowland, an extra- 

 ordinary degree of perfection has been attained. It was many 

 years ago — nearly twenty-five, I am afraid — that I first began 

 experiments upon the photographic reproduction of these 

 divided gratings, each in ilself the work of great time and 

 trouble, and costing a good deal of money. At that time the 

 only gratings available were made by Nobert, in Germany, of 

 which I had two, each containing about a square inch of ruled 

 surface, one of about 3000 lines to the inch, and the other o( 

 about 6000. It happened, accidentally, that ihe grating with 

 3000 lines was the better of the two, in that it was more 

 accurately ruled, and gave much finer definition upon the solar 

 spectrum ; the 6000-line grating was brighter, but its definition 

 was decidedly inferior ; so that both had certain advantages, 

 according to the particular object in view. 



If it conies to the question of how to make a grating by 

 photography, probably the first idea to occur to one would be 

 that it might be a comparatively simple matter to make a grating 

 upon a large scale, and then reduce it by photography ; but if 

 one goes into the figures, the project is not found so promising. 

 Take, for instance, a grating with 10,000 lines to the inch ; 

 if you magnified that, say, 100 times, your lines would then 

 be 100 to the inch ; if you magnified it 1000 times, they 

 would still be 10 to the inch, and that would be a convenient 

 size, .so far as interval between the lines was concerned ; but 

 think what would be the area required to hold a grating magni- 

 fied to that extent. By the time you have magnified the inch 

 by 100 or 1000, you would want a wall of a house or of a 

 cathedral to hold the grating. If the problem were proposed of 

 ruling a grating with 6000 lines to the inch, with a high degree 

 of accuracy, it would be easier to do it on a microscopic scale 

 than upon a large scale, leaving out of consideration the difficulty 

 of reproducing it. And those difiiculties would be insuperable, 

 because, although with a good microscopic object-glass it would 

 be easy to photograph lines which would be much closer 

 together than 3000 or 6000 to the inch, yet that could only be 

 achieved over a very small area of surface — nothing like a square 

 inch ; and if it were required to cover a square inch with lines 

 6000 to the inch, it would be beyond the power, not only, I 

 believe, of any microscope, but of any lens that was ever made. 

 So that that line of investigation does not fulfil the promise 

 which at first it might appear to give ; and, in fact, there is 

 nothing simpler or better than to copy the original ruled by a 

 dividing engine, by the simple process of contact printing. 



For this purpose some precautions are required. You must 

 use very flat glass, by preference it should be optically worked, 

 although very good results may be obtained on selected 

 pieces of ordinary plate. Of course, no one would think of 

 making such a print by difl'used daylight ; but the sun itself, or 

 a point of light from any suitable source, according to the nature 

 of the photographic process which is adopted, permits quite well 

 of the reproduction of any grating of a moderate degree of fine- 

 ness. I have used almost all varieties of photographic processes 

 in my time. In the days when I first worked, the various dry 

 collodion processes were better understood than they are now ; 

 the old albumen process was extremely suitable for such work 

 as this, on account of the almo.st complete absence of structure 

 in the film, and the very convenient hardness of tlie surface, 

 which made the finished re.sult compar.itively little liable to in- 

 jury. I used with success the dry collodion processes, the tannin 

 process among others, and .also some of the direct printing 

 methods, such as the coUodio-chloride. The latter method, 

 worked upon glass, gave excellent results, particularly if the 

 finished print was treated with mercury in the way commonly 



KO. 1397, VOL. 54] 



used for intensification, except that, in the treatment of a grating 

 with mercury, it is desirable to slop at the mercury, and not to 

 go on to the blackening process used in the intensification of 

 negatives. From the visual point of view, the grating, after in- 

 tensification—if one may use the term— with mercury, looks 

 much less intense than before, but, nevertheless, the spectra seen 

 when a point or slit of light is looked at through the grating 

 become very much more brilliant. 



I used another process at that time, more than twenty years 

 ago, which gave excellent results, but had not the degree of cer- 

 tainty that I aimed at, namely, a bichromaied gelatine process, 

 similar to carbon printing, except that no pigment was employed. 

 A glass plate was simply coated with bichromated gelatine of a 

 .suitable thickness — and a good deal depended ujion hitting that 

 off correctly ; if the coating was too thin the grating showed a 

 deficiency of brightness, whereas, if it was too thick, there 

 might be a difficulty in getting it sufficiently uniform and smooth 

 on the surface. However, I obtained excellent gratings by that 

 process, most of them capable of showing the nickel line between 

 the two well-known sodium or I) lines in the solar spectrum, 

 when suitably examined. The coUodio-chloride process was 

 comparatively slow, and bichromated gelatine required two or 

 three minutes' exposure to sunlight to produce a proper effect ; 

 but for the more sensitive developed negative processes a very 

 much less powerful light or a reduced exposure was needed. 



The performance of the copies was quite good, and, except 

 where there was some obvious defect, I never could see that 

 they were worse than the originals ; in fact, in respect of bright- 

 ness it not unfrequently happened that the copies were far 

 superior to the originals, so that in many cases they would be 

 more useful. I do not mean by thai, however, that I would 

 rather have a copy than an original if any one wanted to make me 

 a present. There seems to be some falling off in copies ; .so that 

 they cannot well be copied again, and if you want to work upon 

 spectra of an extremely high order, dispersed to a great extent 

 laterally from the direct line, a copy would not be satisfactory. 

 The reproduction of gratings on bichromated gelatine is easily 

 and quickly accomplished ; there is only the coating of the glass- 

 over-night, rapid drying to avoid crystallisation in the film, 

 exposure, wa.shing, and drying. In order to get the best effect 

 it is usually desirable to treat the bichromated copies with hot 

 water. It is a little difficult to understand what precisely 

 happens. All photographers know that the action of light upon 

 bichromated gelatine is to produce a comparative insolubility of 

 the gelatine. In the carbon process, and m.iny others in which 

 gelatine is used, the gelatine which remains soluble, not having 

 been sufficiently exposed to light, is fairly washed away in the 

 subsequent treatment with warm water ; but for that effect it is 

 generally necessary to get at the back of the gelatine film, 

 because on its face there is usually a layer which is so insoluble 

 as not to allow of the washing away of any of the gelatine to be 

 found behind. But in the present case there is no question of 

 transferring the film, which remains fixed to the glass, and 

 therefore it is difticult to see how any gelatine could be dissolved 

 out. However, under the action of water, the less exposed 

 gelatine no doubt swells more than that which has received 

 more exposure and has thus lost its affinity for water ; and while 

 the gelatine is wet it is reasonable that a rib-like structure should 

 ensue, which is what would be required in order to make a 

 grating, but when the gelatine dries, one would suppose that all 

 would again become flat, and indeed that happens to a certain 

 extent. The gratings lose a great deal of intensity in drying, 

 but, if properly treated with warm water, the reduction does not 

 go too far, and a considerable degree of intensity is left when the 

 photograph is dry. 



Although it belongs to another branch of the subject, a woid 

 may not be out of 'place as to the accuracy with which the 

 gratings must be made. It seems a wonderful thing, at first 

 sight, to rule 6000 lines to an inch at all, if you think of the 

 smallest interval that you can readily see with the eye, perhaps 

 one-hundredth of an inch, and remember that in these gratings 

 there are sixty lines in a space of one-hundredth of an inch, 

 and all disposed at rigorously equal intervals. Those familiar 

 with optics will understand the importance of extreme accuracy 

 if I give an illustration. Take the case of the two sodium lines 

 in the spectrum, the D lines ; they differ in wave-length by 

 about a thousandth part ; the dispersion— the extent to which 

 the light is separated from the direct line— is in proportion to the 

 wave-length of the light, and inversely as the interval between 

 the consecutive lines on the grating ; so that, if we had a grating 



