152 Professor G. Lippmann [April 17, 



rently the corresponding three positives, are obtained interwoven on 

 one and the same plate. The three-coloured method can give a very- 

 good approximation to the truth, and has probably a great future 

 before it. We may call it, nevertheless, an indirect method, since the 

 colours are not generated by the action of light, but are later supplied 

 by the application of aniline dyes or other pigments. Moreover, the 

 choice of these pigments, as well as of the coloured screens through 

 which the negatives have been obtained, is in some degree an arbitrary 

 choice. 



The third and latest method by which colour photography has 

 been realised is the interferential method, which I published in 1891, 

 and the results of which I beg to lay before you this evening. It 

 gives fixed images, the colours of which are due to the direct action of 

 the luminous rays. 



For obtaining coloured photographs by this method, only two 

 conditions are to be fulfilled. We want (1) a transparent grainless 

 photographic film of any kind, capable of giviug a colourless fixed 

 image by the usual means ; and (2) we want a metallic mirror, placed 

 in immediate contact with the film during the time of exposition. 



A mirror is easily formed by means of mercury. The photo- 

 graphic plate being first enclosed in a camera slide, a quantity of 

 mercury is allowed to flow in behind the plate from this small 

 reservoir, which is connected with the slide by a piece of india-rubber 

 tubing.* The slide is then adapted to the camera, and the action 

 of light allowed to take place. After exposure the slide is sepa- 

 rated from the camera, the mercury reservoir lowered so as to 

 allow the mercury to flow back into it; the photographic plate is 

 then taken out, developed and fixed. When dry, and examined by 

 reflected light, it appears brilliantly coloured. 



The sensitive film may be made either of chloride, iodide or 

 bromide of silver, contained in a substratum either of albumen, col- 

 lodion or gelatine. The corresponding developers, either acid or 

 alkaline, have to be applied ; the fixation may be cyanide or bromide 

 of potassium. All these processes I have tried with success. For 

 instance, the photograph of the electric spectrum now projected before 

 your eyes, has been made on a layer of gelatiuo-bromide of silver, 

 developed with amidol, and fixed with cyanide of potassium. 



As you see, bright colour photographs may be obtained without 

 changing the technique of ordinary photography: the same films, 

 developers and fixators have to be employed ; even the secondary 

 operations of intensification and of isochromatisation are made use 

 of with full success. The presence of the mirror behind the film 

 during exposure makes the whole difference. From a chemical point 

 of view nothing is changed, the result being a deposit of reduced 

 silver left in the film, a brownish, colourless deposit. And yet the 



* The g;lass of the photographic plate has to be turned towards the objec- 

 tive, the film in contact with the metallic mirror. 



