August i, 1907] 



jVA TURE 



317 



SIXGLE.PLATE COLOUR-PHOTOGRAPHY. 



THE desire has often been expressed to have a 

 sensitive plate that miijht be exposed in any 

 ordinary camera, and that would yield what has so 

 often been called a photograph " in natural colours." 

 Such plates are now on the market in France, and 

 will doubtless be obtainable in this country as soon as 

 the makers are able to meet the demand for them. 

 It has taken the enterprising firm of Messrs. Lumiere 

 more than three vears to perfect their invention and 

 reduce the manufacture of the plates into a system 

 suitable for the factory. 



Colours are reproduced by these plates only in that 

 limited sense which applies to all three-colour pro- 

 cesses. The natural colours are imitated by tints, 

 which, if successfully produced and seen by a suitable 

 light, are not distinguishable by the unassisted eye 

 from the original. The accuracy of the imitation 

 depends on the choice of the dyes used, and also on 

 the colour-sensitiveness of the emulsion taken in 

 conjunction with the compensating screen used to 

 reduce its excessive sensitiveness to the more re- 

 frangible light that characterises all photographic 

 plates. 



In the ordinary methods of three-colour photography 

 the three colours are separately photographed, using 

 coloured media that transmit only the light required, 

 a print from each negative is obtained in its proper 

 colour, and the three prints are superposed. For one 

 plate to contain in itself the necessities for such a 

 process it is obvious that its surface must be divided 

 among the three colours, and that the separate 

 patches of each must be so small that, as ordinarily 

 viewed, thev are not distinguishable, must be com- 

 parable, in fact, to the lines in a wood cut or the 

 dots or grain in a photomechanical print. Further, 

 if the print is to be complete in itself, the three colours 

 must be an integral part of it, and not, as in Prof. 

 Jolv's method, form a separate "viewing screen." 



The new plates fulfil these conditions. The coloured 

 grained screen that has the three colours in invisibly 

 small patches, forming a tricolour mosaic with the 

 three colours so proportioned that the general impres- 

 sion thev give is a neutral grey, is obtained by means 

 of starch granules. These are selected of fairly uni- 

 form size, and dyed in separate quantities red, green, 

 and violet. The three lots are mixed as thoroughly 

 as possible, and in such proportions that no colour 

 predominates, and spread upon glass as a film one 

 layer thick. The interstices between the rounded 

 granules are filled up by pressing and more or less 

 crushing the grains, an improvement on the original 

 method of filling them up with a black pigment. 

 This three-coloured irregular mosaic is varnished, and 

 a specially sensitised emulsion is spread on the top 

 of it. The plate is then ready for the camera. 



The exposure is made with a suitable colour screen 

 at the lens so that the red, green, and violet lights 

 mav act upon the plate in their proper proportions 

 in spite of the want of orthochromatism of the emul- 

 sion. The glass side of the plate is presented towards 

 the lens, so that the light that forms the image passes 

 first through the layer of dyed starch granules. The 

 sensitive layer will obviously be affected behind each 

 coloured granule so far as the light from the object 

 is of the same colour as the granule. After develop- 

 ment the image has to be reversed, or changed from 

 the negative first produced into a positive. Therefore, 

 instead of fixing in the ordinary way, the metallic 

 silver image is dissolved out by an acid oxidising solu- 

 tion, and the remaining silver bromide is reduced to 

 the metallic state bv a developer. Intensification may 

 be necessary. The result is a three-coloured grained 



NO. 1970, VOL. 76] 



transparency in which the truth of the colours depends 

 upon the conditions stated above. 



It is obvious that such plates must be comparatively 

 costlv, but then only one is required, while some 

 methods of colour photography need six, or even more. 

 The reversal of the image is more trouble than simple 

 fixing, but only one plate has to be dealt with instead 

 of many. The image is granulated, while other 

 methods give results free from grain. It is therefore 

 impossible to say much as to the practical advantage 

 of the me.thod until the plates can be put to actual 

 use. There can, however, be very little doubt that 

 this method, or a modification of it, has a future of 

 usefulness, and no doubt at all as to the ingenuity of 

 the idea that has given rise to it, and the admirable 

 perseverance that has overcome innumerable difficulties 

 in practically working it out. C. J. 



CENTENARY OF THE GEOLOGICAL SOCIETY. 



IN September next the Geological Society will cele- 

 brate its hundredth birthday. In honour of this 

 interesting occasion preparations have for some time 

 been in progress. Invitations to the celebration have 

 been issued to all the foreign members and foreign 

 correspondents of the society ; the various geological 

 survevs all over the globe, universities having chairs 

 of geologv or mineralogy, scientific academies, socie- 

 ties and museums at home and abroad have been 

 invited to send delegates to London. The large 

 number of acceptances already received include the 

 names of many of the most distinguished geologists 

 of the present day, both in the old and the new world. 



It has been arranged that a series of excursions to 

 various parts of this country shall take place before 

 the centennial meeting, under the conduct of fellows 

 of the society conversant with the geology of the 

 several selected districts. These excursions will begin 

 on Wednesday, September iS, and the excursionists 

 will all be back in London by the evening of .September 

 25. The celebration of the centenary, which will 

 extend over three days, will begin on Thursday, Sep- 

 tember 26, at II o'clock, in the Hall of the Institution 

 of Civil Engineers, when the chair will be taken by 

 Sir Archibald Geikie, who has been elected president 

 of the society for' the second time in order that he 

 mav preside on this occasion. The foreign members 

 and foreign correspondents, and the delegates from 

 institutions at home and abroad, will then be received 

 bv him, and will present their addresses. In the 

 afternoon, at 3 o'clock, in the same hall, the president 

 will deliver an address, while in the evening a banquet 

 will be given by the society to its colonial and foreign 

 guests. 



Friday, September 27, will be chiefly devoted to 

 visits to museums, galleries, &c., concluding with an 

 evening reception. On Saturday, September 28, short 

 excursions have been projected to places of geological 

 interest within easy reach of London. On Monday, 

 September 30, the visitors will be divided into two 

 sections, one of which will go to Oxford, the other 

 to Cambridge. It is understood that the universities 

 will confer honorary degrees on some of the more dis- 

 tinguished geologists from beyond the seas, and that 

 college hospitalitv will be as abundant and hearty as 

 usual, while those visitors w-ho may still have energy 

 enough left for field-work will be taken on geological 

 excursions from both the university towns. This well- 

 planned combination of scientific intercourse with 

 social pleasure can hardlv fail to have a lasting effect 

 in forming and confirming friendships by bringing 

 the geologists of many different countries into close- 

 personal relations with each other. 



