6i8 



NA TURE 



[April 2i^, 1896 



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 

 otherwise provide for a mirror being in contact with it. The 

 presence of the mirror changes the propagated waves into 

 standing waves. The reflected 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 photographic 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 TTrTTTTr millimetres, half the wave-length in the above 

 quoted distance of TTrfirTr 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 violet rays, 

 and making these alone apparent, when illumined 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 thick- 

 ness 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 up one-tenth of a millimetre. Lord Ray- 

 leigh, in 1887, has proved ii priori SksaX such a system is specially 

 adapted to reflect the corresponding 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 interference, 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 

 colours 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 every other position we see nothing but a 

 colourless negative. Now, as you are aware, the colours of 

 pigments are seen in any direction. By projecting again a photo- 

 graph of the spectrum, and turning it to and fro, I can show you 

 that the colours are visible only in one direction. 



Thirdly, if we change the incidence of the illuminating rays, 

 that is, if we look at the plate first in a normal direction, then 

 more and more slantingly, we find that the colours change with 

 the incidence exactly as they do in the case of soap-bubbles, or 

 of Newton's rings ; they change according to the same law, and 

 for the same reasons. The red end of the spectrum turns 

 successively to orange, yellow, green, blue, and violet. The 

 whole system of colours, the image of the spectrum, is seen to 

 move down into the part impressed by the infra-red. This is 

 what we expect to happen with interference colours, and what 

 again we cannot obtain with pigments. 



P^ourthly, if while looking at the film normally, we suffer it to 

 absorb moisture — this can be done by breathing repeatedly on 

 its surface — ^we see that the colours again change, but in an 

 order opposite to that above described. Here the blue end of 

 the spectrum is seen to turn gradually green, yellow, orange, 

 red, and finally infra-red, that is, invisible. The spectrum this 

 time seems to move up into the ultra-violet part of the improved 

 film. By suff"ering the water to evaporate, the whole image 

 moves back into its proper place ; this experiment may be 

 repeated any number of times. 



The same phenomenon may be obtained with Newton's 

 apparatus, by slowly lifting the lens out of contact with the 

 plane surface. The explanation is the same in both cases. The 

 gelatine swells up when imbibing moisture. If we consider, for 



NO. 1383. VOL. 53] 



instance, the violet of the spectrum, the small intervals between 

 the strata, corresponding to violet rays, gradually swell up to the 

 values proper for green, and for red, and for infra-red ; green, 

 then red, then infra-red, are therefore successively reflected. 



We will wet this photograph of the spectrum with water, 

 project it on the screen, and watch the colours coming back in 

 the order prescribed by theory. 



It is necessary to use a transparent film, since an opaque one, 

 such as is commonly in use, would hide the mirror from view ; 

 the sensitive substance must be grainless, or, at least, the grains 

 must be much finer than the dimensions of the strata they are 

 intended to form, and therefore wholly invisible. The pre- 

 paration of transparent layers gave me at first much troulile ; 

 I despaired for years to find a proper method for making them. 

 The method, however, is simply thus : if the sensitive substance 

 (the silver bromide, for instance) be formed in presence of a 

 sufficient quantity of organic matter, such as albumen, gelatine, 

 or collodion, it does not appear as a precipitate ; it remains in- 

 visible ; it is formed, but seems to remain dissolved in the 

 organic substratum. If, for instance, we prepare a film of 

 albumeno-iodide in the usual way, only taking care to lessen 

 the proportions of iodide to half per cent, of the albumen, we 

 get a perfectly transparent plate, adapted to colour photography. 



We want now to go a step further. It is very well for 

 physicists to be contented with working on the spectrum, since 

 that contains the elements of every compound colour ; but we 

 all desire to be able to photograph other objects than the 

 spectrum — common objects with the most compound colours. 

 We have again but to take theory as a guide, and that tells us 

 that the same process is able to give us either simple or com- 

 pound colours. We have then to take a transparent and 

 correctly isochromatised film, expose it with its mercury backing, 

 then develop and fix it in the usual way ; the plate, after dry- 

 ing, gives a correct coloured image of the objects placed before 

 the camera. Only one exposure, only one operation is 

 necessary for getting an image with every colour complete. 



A plausible objection was ottered at first to the possibility of 

 photographing a mixture of simple colours. The objection was 

 this : a ray of violet gives rise to a set of strata separated by a 

 given interval ; red light produces another set of strata with 

 another interval ; if both co-exist, the strata formed by the red 

 are sure to block out here and there the intervals left between 

 the strata formed by the violet. Is it not to be feared that one 

 fabric will be blurred out by the other, and the whole eff'ect 

 marred ? The confusion would be still worse if we consider the 

 action of white light, which contains an infinity of simple com- 

 ponents ; every interval here is sure to be blocked up. 



Mathematical analysis, however, shows this objection to be 

 unfounded ; we have great complexity, but not confusion. 

 Every compound ray, both coloured and white, is faithfully 

 rendered. As an experimental proof of this, we will project on 

 the screen photographs of very different objects, namely, stained- 

 glass windows, landscapes from nature, a portrait made from 

 life, and vases and flowers. 



That the colours here observed are due to interference, and 

 not to the presence of pigments, can be shown in the same way 

 as with the spectrum. Here, again, we observe that the colours 

 are visible only in the direction of specular reflection, that they 

 change with the angle of incidence, that they change and dis- 

 appear by wetting, and reappear by drying. Pigments remain 

 equally visible and unaltered in colour under every incidence. If 

 we attempted to touch up one of our photographs with oil or 

 water-colours, the adulterated place would stand out on a colour- 

 less background by merely obscuring by diff'used light. It is 

 therefore impossible either to imitate or touch up a colour 

 photograph made by the above described interferential method. 



THE INFLUENCE OF ATMOSPHERIC AND 



OCEANIC CURRENTS UPON TERRESTRIAL 



LATITUDES} 



A LTHOUGH the following theorem should be implicitly 



contained in the formula for the rotation of a spheroid 



carrying a fluid on its surface, I have nowhere seen it explicitly 



stated. 



Theorem. — Let an unconstrained, rigid sphere, with equal 

 moments of inertia, be in a state of free rotation : 



Let this sphere bear on its surface a sheet or sheets of con- 

 tinuous movable matter : 



•I Reprinted from the Astronomical Journal, No. 371, April 6. 



