July 2, 1891] 



NATURE 



95 



The subject of interference receives more detailed 

 treatment in chapter v., the interference of direct and 

 reflected waves, and the theory of Newton's rings, being 

 specially dealt with. It is not till we come to the sixth 

 chapter that we are introduced to the main subject of the 

 brochure. The principle which guided M. Lippmann in 

 his experiments is well and tersely given. Imagine a 

 plane metallic mirror with its reflecting surface coated 

 with a transparent, homogeneous film of a silver haloid 

 in albumin or collodion. Supposing a coloured ray of 

 definite wave-length to fall on such a film, the undulations 

 would traverse the transparent sensitive film, and being 

 reflected from the polished surface of the mirror, and 

 meeting the incident waves, would produce interference. 

 The space in front of the mirror would thus be occupied 

 by parallel planes alternately light and dark, and separated 

 by half wave-lengths, i.e. by spaces of 1/4,000,000 of a 

 millimetre. There is therefore ample space, even within 

 the thickness of the film, for several of these planes of 

 interference. On development, the planes corresponding 

 to the light intervals would alone give films of metallic 

 silver, while the dark intervals would remain unaffected. 

 On fixing, there would thus be left in the film a series of 

 parallel films of metallic silver separated by half wave- 

 lengths. Any pair of such films constitute a thin plate in 

 the Newtonian sense, and will give by interference a 

 colour corresponding to that which produced the original 

 deposition of the films when viewed by reflected light. 



To realize the foregoing principle experimentally, M. 

 Lippmann has found it necessary to use dry films of 

 collodion, or albumin, or gelatine sensitized by immersion, 

 as in the old wet collodion process : emulsions are granular 

 and opaque, and contain particles which are gross in 

 comparison with the half wave-length of a spectrum 

 colour, and cannot be used. Moreover, it has not 

 been found practicable to coat the reflecting surface of 

 the mirror directly with the sensitive film, because the 

 free iodine tarnishes the silver and destroys its reflecting 

 power. This difficulty has been surmounted by making 

 the coated glass plate one side of a shallow trough with 

 parallel sides filled with mercury, the coated side being 

 inwards, and in close contact with the mercury. The 

 conditions for reflection and interference are thus ful- 

 filled. The image of the spectrum is focussed on a glass 

 plate with a ground surface, which is temporarily fixed to 

 the side of the cell or trough in the same position as that 

 occupied by the sensitive plate, i.e. with the ground 

 surface inwards. After focussing, the ground glass is 

 removed, and the sensitive plate substituted for it in 

 the position described. 



The spectrum was produced by an electric arc light of 

 800 candle-power, and the time of exposure for the 

 different parts of the spectrum was regulated by inter- 

 posing cells with coloured solutions, beginning with a 

 solution of helianthin which transmits only the red and 

 yellow, then replacing this by a cell of potassium dichro- 

 mate which transmits the red, yellow, and green, and 

 then finally exposing for a few seoonds without any 

 screen, so as to impress the blue and violet. The whole 

 time of exposure varies, according to the sensitiveness of 

 the film, from half an hour to two hours. The details of 

 development and fixing are given by M. Berget, and do 

 not differ fundamentally from the ordinary methods. 

 NO. I 131, VOL. 44] 



The finished image, when dry, shows the spectrum 

 colours by reflected light with metallic brilliancy, and 

 as the colours are purely optical, depending only on 

 reflection and interference, they are permanent. As the 

 author points out, it is certainly a marvellous tribute to 

 the fidelity of the photographic method that a series of 

 laminae of metallic silver separated by intervals of only 

 about 1/4,000,000 of a millimetre should retain their 

 positions with optical accuracy during the processes of 

 fixing and development. 



There can be no doubt — as will be admitted by all who 

 have seen the results — that M. Lippmann is to be con- 

 gratulated on having made a most important advance in 

 the methods of photochromy. How far his experiments 

 go towards the realization of the great problem of photo- 

 graphing objects in their natural colours is a question 

 quite distinct from his present achievement. M. Berget 

 tells us that satisfactory reproductions of coloured glasses 

 illuminated from behind by the electric light have been 

 obtained, but this is only a very little step in the desired 

 direction. 



" Que reste-t-il h. faire pour rendre absolument usuel le 

 procddd photochromique de M. Lippmann ? " There re- 

 mains a great deal ! Not the least of the requirements 

 is a transparent sensitive film equally sensitive to every 

 colour of the spectrum, and sufficiently sensitive as a 

 whole to enable the impression to be secured with a 

 moderate exposure, instead of 30 to 120 minutes. Till 

 this is accomplished we are not much nearer the solution 

 of the problem of photography in natural colours than we 

 were before. M. Berget speaks hopefully of the prospects 

 in this direction, and we wish every success to his anti- 

 cipations. But it is no detraction from the merit of M. 

 Lippmann's results if these have no immediate bearing 

 on practical photographic processes. As a triumph of 

 physical science these experiments will live. 



" Cest aussi un triomphe pour la science fran^aise, car 

 ce mode de reproduction des couleurs du spectre h. I'aide 

 des lames minces limitdes par des plans d'argent constitue 

 une matdrialisation, rdalisde par un savant frangais, de 

 ces ondes lumineuses congue pour le premiere fois par le 

 puissant genie d'un autre Frangais illustre : j'ai nomm^ 

 Au^ustin Fresnel." 



With this patriotic outburst M. Berget concludes his 

 pamphlet, and the compatriots of Niepce and Daguerre 

 may well be gratified with this latest emanation from the 

 physical laboratory of the Sorbonne. 



R. Meldola. 



OUR BOOK SHELF. 



Geometry of Position. By R. H. Graham, Author of 



" Graphic and Analytic Statics." (London and New 



York : Macmillan and Co., 1891.) 

 This work essays to fill an existing want by providing an 

 EngHsh text-book on the important subject of geometry of 

 position in relation to graphical statics. 



The author gives an introductory chapter on anharmonic 

 pencils and ratios, followed by an interesting chapter on 

 projective conies, and devotes the remainder of the book 

 to the application of graphic methods to statical problems, 

 including, amongst others, the discussion of Maxwell's 

 theory of reciprocal figures. 



The chapter on anharmonic pencils and ratios would 

 have been considerably improved by the introduction, at 

 the beginning, of more definitions and e.xplanations of the 



