17S 



KNOWLEDGE. 



[August 2, 1897. 



manent, but I can find no trace of his having claimed so 

 much ; no permanent works are known to exist, and it 

 is known that many of his pictures were very fleeting. 

 Poitevin, in 1868, stated that the coloured image could 

 be "fixed" by means of sulphuric acid ; and I believe he 

 gave to the late J. Traill Taylor certain of these coloured 

 pictures, which, "kept in a drawer without any special 

 precautions," retained their colours for (at least) several 

 years. But Mr. Taylor found it impossible to fix similar 

 impressions by any application of sulphuric acid that he 

 could make, and I believe that all other experimenters 

 have been equally unsuccessful. 



In March, 1890, Franz Veresc, of Klausenberg, exhibited 

 at the Photographic Institute, Vienna, results on both 

 glass and paper, which were highly praised at the time, and 

 believed to be of great promise. The newspapers of the 

 whole world rang with his fame, but nothing further has 

 been heard on the subject. 



The next important announcement, and one that has 

 been fully justified, was made in June, 1891, when Alphonse 

 Berget published, " Photographic des Couleurs par la 

 Methode interferentielle de M. Lippmann." A preliminary 

 announcement had been made in March of the same year 

 by M. Lippmann himself. The process is totally different, 

 in theory and in practice, from all others, and from the 

 scientific point of view is perfect. It has, however, certain 

 serious practical disadvantages, which I shall shortly 

 mention. The method is based on the " interference " of 

 light waves, and depends upon the idea that if light waves 

 are reflected back along their original path in such a way 

 that they twice pass through a sensitive film, the silver 

 will be deposited in laminas in the thickness of the film ; the 

 distance between the laminre being governed by the wave- 

 length of the light. Thus, every portion of the film has an 

 arrangement of its silver particles in definitely placed 

 strata, and forms, after development and fixation, a light- 

 filter allowing only light of the same wave-length which 

 deposited the silver to be reflected through it. In practice 

 the sensitive plate is made the front of a trough of 

 mercury, so that the light passing through the film strikes 

 the mercury and is reflected along the same path. The 

 pictures obtained are of great beauty and wonderful 

 fidelity to Nature, the difficulty being that they can only 

 be seen when the plates are held at a certain angle. 

 Practically the most satisfactory way of viewing the 

 results is by casting upon them the light of a powerful 

 optical lantern, which is reflected from the surface of the 

 picture on to a white screen — and this is hardly the 

 method which the general public needs. 



As demand always creates a supply of some kind there 

 has been no lack, latterly, of methods, ostensibly more or 

 less simple, for photographing in colours. I must admit 

 that many people who have tried these methods seem 

 unable to obtain any result, yet as they have been read 

 before reputable Continental societies it seems scarce 

 likely that they are mere frauds. A type of several is 

 a paper given before the Socicte Francaise de Photo- 

 graphic by j\r. de Saint-Florent. He takes celloidin 

 (collodio-chloride) paper, exposes it to sunlight until it 

 assumes a reddish-black colour, then soaks it for ten 

 minutes in a bath of — 



Alcohol (Beaume 35") ... Bounces 



G-lyceriue 2 drams 



Tincture of Iodine (2i%) ... 2 drams 



Ammonia (■S8C) ... ... 6 drops 



Dry in dark room, then expose under coloured transparency 

 in direct sunlight for about an hour, when the colours will 

 appear. Fix in a bath of hyposulphite of soda (six per 

 cent, to ten per cent.). In the fixing the colours become 



brilliant at first, then totally fade to a pale lemon yellow ; 

 but if the print is then withdrawn, washed rapidly, and 

 dried in the sun or before a bright fire, the colours will 

 reappear in full brilliancy, and be permanent. One of 

 my correspondents says that this method results in the 

 deposit, in the first bath, and on the paper, of a 

 precipitate of iodide of nitrogen, which is a sufficiently 

 sensitive fulminate to take fire from rubbing with the 

 fingers when the paper is dry. 



Another method was given quite recently, before the 

 Acadomie des Sciences of Paris, by A. Graby. The paper 

 is a lengthy one, complete formulaj are given, and it has 

 the advantage of being mainly based upon suggestions 

 already made by other workers. The whole is given (in 

 a condensed translation) in the Photoiiram for February 

 last, and I cannot well give it in the present limited 

 space. It is based upon work with the various coloured 

 subchlorides of silver, and the author begins by preparing 

 a film in which, simultaneously, we have red and blue 

 subchlorides and a yellow chromate of silver. He says, 

 to quote the translation : — 



" Under the influence of blue light the blue subchloride 

 is not sensitive, and remains the same ; but the red, if 

 mercuric chloride is present, takes up chlorine and is 

 converted into blue. If chromic acid be present also, the 

 yellow becomes oxidized to blue. 



" Under yellow light the chromic acid is not affected, 

 but the blue and red — especially the former — are bleached, 

 losing their chlorine and amalgamating with mercury. 



" Under red light the red subchloride remains un- 

 changed, but the blue is changed by the red-orange rays 

 into the red sub-chloride." 



Thus we see that each light-ray ignores the pigment of 

 its own colour, and converts to its own colour the pigments 

 that differ. All this, says M. Graby, is old ground ; and 

 what he claims as new is a method of removing the old 

 difficulty which caused the whites to be represented by 

 black, and the lighter greys by dark greys, etc., owing to 

 the production of a great amount of pigment under white 

 light. To overcome this difficulty the author uses a 

 gelatine film which, at the outset, is insoluble, but which 

 is rendered soluble by the action of light, the most com- 

 plete solubility occurring where white light has acted. 

 The print is then " developed " in warm water, as in the 

 carbon process, in which the gelatine is washed away in 

 proportion to its solubility. 



Perhaps I have dwelt too long upon these methods, and 

 left too little space for the recently boomed results of 

 Bennetto and Chassagne. In point of time, Bennetto's 

 announcement came a year or more before Chassagne's, and 

 in quality his work is immensely superior. Of Bennetto's 

 method we know nothing. All the pictures he has shown 

 hitherto are transparencies on glass, and he will not allow 

 them to be handled by anyone else, even for the purpose of 

 placing them in the optical lantern. The results are 

 wonderful — as near perfection as we need hope to come. 



Chassagne's colour pictures, on the other hand, though 

 wonderful as purely photographic productions, have the 

 appearance of ordinary silver prints tinted with aniline 

 dyes. Of the method we know something. A negative is 

 made, in the ordinary way, on a plate prepared with 

 a " special solution." This is developed in the ordinary 

 way and produces an ordinary negative. From this a 

 print is made on paper prepared with the same " special 

 solution," which gives an ordinary print, which is ordinarily 

 toned and fixed. The print is then immersed, successively, 

 in three baths of dye, when it absorbs each in the portions 

 needing such colour. The manipulations — exposing, de- 

 veloping, and printing — have been carried out by Sir Henry 



