154 Professor G. Lippmann [April 17, 



the film more or less strongly, but leaves no record of its wave-length, 

 of its particular nature or colour, every trace of its passage being swept 

 out of form by reason of its swift displacement. The impression 

 therefore remains both uniform and colourless. Things change, 

 however, as soon as we pour in mercury behind the plates, or other- 

 wise provide for a mirror being in contact with it. The presence of 

 the mirror changes the propagated waves into standing waves. The re- 

 flected ray is, namely, thrown back on the incident ray, and interferes 

 with its motion, both rays having equal and opposite velocities of 

 propagation. The result is a set of standing waves — that is, of waves 

 surging up and down, each in a fixed plane. Each wave impresses the 

 sensitive film where it stands, thus producing one of these photo- 

 graphic strata above alluded to. The impression is latent, but comes 

 out by photographic development. Of course the distance between 

 two successive strata is the distance between two neighbouring waves ; 

 this, theory shows, is exactly half the wave length of the impressing 

 light. In the case of violet, for instance, the wave-length being 

 ^o ^oQ millimetres, half the wave-length in the above-quoted distance 

 of ToJoo^ millimetres ; this, therefore, is at the same time the interval 

 between two standing waves, in the case of violet light the interval 

 between two successive photographic strata, and at last it is the interval 

 required to exist, according to Newton's theory, for the said strata 

 reflecting voilet rays, and making these alone apparent when illumi- 

 nated by white light. 



The colours reflected by the film have the same nature and origin 

 as those reflected by soap-bubbles or Newton's rings ; they owe their 

 intensity to the great number of reflecting strata. Suppose, for 

 instance, the photographic film to have the thickness of a sheet of 

 paper (one-tenth of a millimetre), the fabric built in it by and for 

 a violet ray is five hundred stories high, the total height making 

 np one-tenth of a millimetre. Lord Eayleigh, in 1887, has proved 

 a priori that such a system is specially adapted to reflect the corre- 

 sponding waves of light. 



How are we now to prove that the above theory is really applicable 

 to the colour photograph you have seen ? How can we demonstrate 

 that those bright colours are due not to pigments, but to the inter- 

 ference, as in the case of soap-bubbles ? We have several ways of 

 proving it. 



First of all, we are not bound to the use of a peculiar chemical 

 substance, such as Becquerel's subchloride of silver ; we obtain colour 

 with a variety of chemicals. We can, for instance, dispense entirely 

 with the use of a silver salt; a film of gelatine or coagulated 

 albumen impregnated with bichromate of potash, then washed with 

 pure water after exposure, gives a very brilliant image of the spectrum. 



Secondly, the colours on the plate are visible only in the direction 

 of specular reflection. The position of the source by which we 

 illumine the photograph being given, we have to put the eye in a 

 corresponding position, so as to catch the regularly reflected rays. In 



