January 9, 1896] 



NATURE 



235 



Wuish hue if held in bkie light. It is likewise possible by 

 proper selection of luminescent salts to produce a selected series 

 <il tints during and after exposure to those rays which are most 

 effective in photography. But such colours cannot be made fixed 

 and permanent. The problem of securing on the photographic 

 plate a faithful and lasting reproduction of the various tints of 

 a spectrum thrown upon it has baffled most of those who 

 grappled with this subject. That it has l)een fully and quite 

 satisfactorily solved cannot yet be affirmed, but the last few 

 years have brought a far nearer approach to success than an 

 •equal number of decades previously. Viewed from the scientific 

 standpoint the goal has certainly been tt)uched, even if com- 

 mercial demands are still made in vain. 



Stationary Light Waves. 



Two quite different methods are to be considered in tracing 

 the recent development of this interesting application of optical 

 principles. The first is originally due to Kecquerel {Am. de 

 Chimie et de Physique (3), p. 451, 1848), but lately, in the 

 hands of Lippmann, it has been improved and brought much 

 nearer to success than by its originator. It depends upon the 

 production of stationary waves of light. Every one is familiar 

 with the formation of stationary waves upon an elastic stretched 

 cord, and with the acoustic exhibition of stationary air waves in 

 a closed tube by Kundt's method of light powders. That 

 similar loops and nodes must be produced under proper con- 

 •ditions by interference of waves of light would appear obviously 

 jxjssible ; and so long ago as 1868 Dr. Zenker " Lehrbuch der 

 Photochromie," Berlin, 1868), of Berlin, explained the photo- 

 graphic reproduction of colour, so far as it had then been 

 accomplished, by reference to stationary light waves. But no 

 definite proof of their production had been brought forward. X 

 few years ago Hertz demonstrated objectively the electro- 

 magnetic waves whose existence had been foretold by Maxwell's 

 genius ; and with suitable apparatus stationary electric waves 

 are now almost as readily made evident as are those of sound. 

 Hertz's brilliant success stimulated his fellow countryman, 

 Otter Wiener, to undertake the apparently hopeless task of 

 producing and studying stationary light waves. Wiener's 

 admirable work {Wiedemann s Annalen, Band xl. 1890, p. 

 203) excited great interest on the continent of Europe, but it 

 has been singularly neglected in England and America. It is 

 worth much more than a passing notice. 



Assume a plane silvered mirror upon which a bundle of rays 

 ■of monochromatic light fall normally so as to be reflected back 

 upon its own path. The superposition of reflected and direct 

 waves causes a system of stationary waves, but under ordinary 

 conditions these are wholly imperceptible. The nodes are formed 

 iipon a series of planes obviously parallel to the reflecting plane 

 at successive distances of a half wave-length. If now we con- 

 sider a plane oblique to the mirror, it will cut these successive 

 nodal planes in parallel lines, whose distance apart will be 

 greater in proportion as the oblique plane approaches parallelism 

 to the mirror. Although a half wave-length of violet light is 

 •<mly si-a-i of a millimetre, it is easy to conceive of the cutting 

 plane forming so small an angle with the mirror that the distance 

 between the parallel nodal lines shall be a thousand times a half 

 wave-length. Such would be the case if the inclination of the 

 cutting plane is reduced to a little less than four minutes of arc. 

 The nodal lines would be ^ of a millimetre apart, and readily 

 callable of resolution if their presence can be manifested at all. 

 Imagine a very thin transparent photographic film to be stretched 

 along the oblique cutting plane, and developed after exposure 

 to violet light as nearly monochromatic as possible. Then the 

 developed negative should present a succession of jmrallel clear 

 and dark lines, corresponding to nodal and anti-nodal tends 

 along the oblique plane, the photographic effect being annihilated 

 along an optical nodal line. 



The realisation of a photographic film thin enough for such 

 an experiment is quite conceivable when we remember that 

 imder the hammer gold is beaten into leaves so delicate that 

 8cKX> of them would be required to make a pile one millimetre 

 thick. By electrochemical deposit, Outerl)ridge [journal of the 

 Franklin Institute, vol. ciii. p. 284, 1877) has made films of 

 gold whose thickness is only ,^iu5xrTr of a millimetre, or vtV of a 

 -wave-length of sodium light. Wiener obtained a perfectly 

 transparent silver chloride film of collodion, whose thickness 

 was about ^'^ of a wave-length of sodium ligbt. This was 

 formed on a plate of glass and inclined at a very small angle to 

 at i>Iane silvered mirror which served as reflector. Erom an 



NO. 1367, VOL. 53] 



electric arc lamp the light was sent through :in ajipropriate slit 

 and prism, so that a selected spectral band of violet fell normally 

 on the prepared plate in the dark room. The developed 

 negative presented the alternate bands, in perfectly regular 

 order, more than a half millimetre apart. X'arious tests were 

 applied to guard against error in interpretation, and the existence 

 of such stationary waves was proved beyond all doubt. 



These waves, moreover, when polarised light was employed, 

 furnished the means of determining the direction of vibration 

 with relation to the plane in which the light is most copiously 

 reflected when incident at the polarising angle, and thus of sub- 

 jecting to experiment the question as to whether the plane of 

 vibration is coincident with this plane of polarisation or is per- 

 pendicular to it. The former of these views was held by Neumann 

 and MacCullagh, the latter by Fresnel. Let a Ijeam of polarised 

 light fall upon the mirror at an angle of about 45°. If the vibra- 

 tions in the incident beam are parallel to the mirror, and hence 

 perpendicular to the plane of polarisation, those of the reflected 

 and incident l)eams will be parallel to each other, and hence 

 capable of interference. But if the vibrations of the incident 

 beams are in a plane identical with that of incidence, and hence 

 in the plane of jjolarisation, the vibrations of incident and re- 

 flected beams are in mutually perpendicular planes, and hence 

 cannot interfere. Wiener obtained interference fringes when 

 the light was polarised in the plane of incidence, while the 

 polarised in the plane perpendicular to this gave no trace of in- 

 terference. The theory of Fresnel was thus confirmed experi- 

 mentally. .\gain, the familiar phenomenon of Newton's rings 

 shows us that en changing media there is a change of phase of 

 the incident light, else the central spot where the two surfaces 

 come into optical contact would be white instead of black. But 

 there has been difference of opinion as to whether this change of 

 phase occurs at the upper surface of the air film, where the light 

 passes from glass to less dense air, or at the lower surface where 

 it passes from air to more dense glass. In the latter event, 

 there should l>e a node at the reflecting surface. Replacing the 

 silvered plane surface by a lens in contact with the photographic 

 film, Wiener obtained circular fringes with no photographic 

 action, at the centre, showing the nodal point to be at the point 

 of contact, and thus again confirming the theory of Fresnel. 



Colour Photography. 



The conditions being now specified under which stationary 

 light waves are produced, let us imagine common instead of 

 monochromatic light to be transmitted normally through a trans- 

 parent sensitive film. Then a variety of stationary interference 

 planes are produced. This is the underlying principle of the 

 process employed by Lippmann in Paris, who, in 1892 {Coniptes 

 rendiis, t. cxiv. p. 961, and t. cxv. p. 575), succeeded in obtain- 

 ing a photograph of the solar spectrum in natural colours. Upon 

 a surface backed with a reflecting mirror of mercury is a silver 

 bromide albumen film, which has been treated with one or more 

 aniline dyes to render it equally sensitive to waves of long and 

 short period, .\fter exposure and development the natural 

 colours are manifested with brilliancy. Apart from the funda- 

 mental principle already expressed, it can scarcely be said that 

 the rationale of the process has yet been very fully and clearly 

 explained. Lippmann recognises the stationary wave systems, 

 with maxima and minima of brightness in the film and corre- 

 sponding maxima and minima of silver deposit. If the incident 

 light is homogeneous, a series of equidistant parallel planes of 

 equal photographic efficiency are produced in the film. If the 

 plate after development is illuminated with white light, then to 

 every point within the film there comes from below a certain 

 amount of reflected energy which is a continuous periodic func- 

 tion of the distance from the reflecting surface. The total re- 

 flected light of any colour becomes then represented by the in- 

 tegral of this jjeriodic function for the entire thickness of the layer. 

 The solution of this integral brings the result that the intensity 

 of the reflected ight decreases with increasing thickness of the 

 layer, approaching zero as a limit, so long as this light is of dif- 

 ferent wave length from the homogeneous light employed for 

 illumination of the plate. Only light of the same wave-length, 

 or of an entire multiple of this, maintains a finite value. A 

 similar consideration applies to each of the hues composing white 

 light. By such mathematical considerations Lippmann (Journal 

 de Physii/ue, p. 97, 1894) reaches the conclusion that the light 

 reflected from the plate must have exactly the same relations of 

 wave-length as that with which the plate was illuminated. 



For the Lippmann photographs, which at first required a very 



