472 



NA rURE 



[Sept. 12, 1889 



to be of use in this manner it may be taken as an axiom —first 

 propounded by the speaker, it is believed — that it must be fugi- 

 tive, or that it must be capable of forming a silver compound. 

 The more stable a dye is the less effective it is. If we take as an 

 example cyanine, we find that it absorbs in the orange and slightly 

 in the red. If paper or collodion stained with this colouring- 

 matter be exposed to the action of the spectrum, it will be found 

 that the dye bleaches in exactly the same part of the spectrum as 

 that in which it absorbs, following, indeed, the universal law I 

 have already alluded to. If a film containing a silver salt be dyed 

 with the same, it will be found that, whilst the spectrum acts on 

 it in the usual manner — viz. darkening it in the blue, violet, and 

 ultra-violet — the colour is discharged where the dye absorbs, 

 showing that in one part of the spectrum it is the silver salt which 

 is sensitive, and that in the other it is the colouring-matter. If 

 such a plate, after exposure to the spectrum, be developed, it will 

 be found that at both parts a deposit of silver takes place ; and 

 further, when the experiment is carefully conducted, if a plate 

 with merely cyanine-coloured collodion be exposed to the spec- 

 trum and bleached in the orange, and after removal to the dark 

 room another film containing a silver salt be applied and then a 

 developer, a deposit of silver will take place where the bleaching 

 has occurred. This points to the fact that the molecules of a 

 fugitive dye, when altered by light, are unsatisfied, and are ready 

 to take up an atom or atoms of silver, and other molecules of 

 silver will deposit on such nuclei by an action which has various 

 names in physical science, but which I do not care to mention. 

 This is the theory which I have always advocated, viz. that the 

 dye by its reduction acts as a nucleus on which a deposit of silver 

 can take place. It met with opposition ; a rival theory which 

 makes the dye an "optical sensitizer" — an expression which 

 is capable of a meaning which I conceive contrary to physical 

 laws — being run against it. The objection to what I may call 

 the nucleus theory is less vigorous than it has been, and 

 its diminution is due perhaps to the more perfect under- 

 standing of the meaning of each other by those engaged 

 in the controversy. To my mind, the action of light on fugitive 

 dyes is one of the most interesting in the whole realm of photo- 

 graphy, as eventually it must teach us something as to the 

 structure of molecules, and add to the methods by which their 

 coarseness may be ascertained. Be the theory what it may, 

 however, a definite result has been attained, and it is now 

 possible to obtain a fair representation of the luminosity of 

 colours by means of dyed films. At present the employment of 

 coloured screens in front of the lens, or on the lens itself, 

 is almost an essential in the method when daylight is 

 employed, but not till some dye is discovered which shall 

 make a film equally sensitive for the same luminosity to the 

 whole visible spectrum will it be possible to makeorthochromatic 

 photography as perfect as it can be made. The very fact that no 

 photograph of even a black and white gradation will render the 

 latter correctly must of necessity render any process imperfect, 

 and hence in the above sentence I have used the expression " as 

 perfect as it can be made." 



The delineation of the spectrum is one of the chief scientific 

 applications to which photography has been put. From very 

 early days the violet and ultra-violet end of the spectrum have 

 been favourite objects for the photographic plate. To secure 

 the yellow and red of the spectrum was, however, till of late 

 years, a matter of apparently insurmountable difficulty ; whilst a 

 knowledge of that part of the spectrum which lies below the 

 red was only to be gained by its heating effect. The introduc 

 tion of the gelatine process enabled the green portion of the 

 spectrum to impress itself on the sensitive' surface ; whilst the 

 addition of various dyes, as before mentioned, allowed the 

 yellow, the orange, and a portion of the red rays to become 

 photographic rays. Some eight years ago it was my own good 

 fortune to make the dark infra-red rays impress themselves on a 

 plate. This last has been too much a specialty of my own, 

 although full explanations have been given of the methods 

 employed. By preparing a bromide of silver salt in a pecu- 

 liar manner one is able so to modify the molecular arrange- 

 ment of the atoms that they answer to the swings of those waves 

 which give rise to these radiations. By employing this salt of 

 silver in a film of collodion or gelatine the invisible part of the 

 spectrum can be photographed and the images of bodies which 

 are heated to less than red heat may be caused to impress them- 

 selves upon the sensitive plate. The greatest wave-length of the 

 spectrum to which this salt is sensitive so far is 22,000 \, or five 

 times the length of the visible spectrum. The exposure for such 



a wave-length is very prolonged, but down to a wave-length of 

 12,000 it is comparatively short, though not so short as that 

 required for the blue rays to impress themselves on a collodion 

 plate. The colour of the sensitive salt is a green blue by trans- 

 mitted light ; it has yet to be determined whether this colour is 

 all due to the coarseness of the particles or to the absorption by 

 the molecules. The fact that a film can be prepared which by 

 transmitted light is yellow, and which may be indicative of colour 

 due to fine particles, together with an absorption of the red and 

 orange, points to the green colour being probably due to absorption 

 by the molecules. We have thus in photography a means of record- 

 ing phenomena in the spectrum from the ultra-violet to a very 

 large wave-length in the infra-red — a power which physicists 

 may some day turn to account. It would, for instance, be a 

 research worth pursuing to photograph the heavens on a plate 

 prepared with such a salt, and search for stars which are nearly 

 dead or newly born, for in both cases the temperature at which 

 they are may be such as to render them below red heat, and 

 therefore invisible to the eye in the telescope. It would be a 

 supplementary work to that being carried out by the brothers 

 Henri, Common, Roberts, Gill, and others, who are busy 

 securing photographic charts of the heavens in a manner which 

 is beyond praise. 



There is one other recent advance which has been made 

 in scientific photography to which I may be permitted to 

 allude, viz. that from being merely a qualitative recorder 

 of the action of light it can now be used for quantitative 

 measurement. I am not now alluding to photographic actino- 

 meters, such as have been brought to such a state of perfection 

 by Roscoe ; but what I allude to is the measurement and inter- 

 pretation of the density of deposit in a negative. By making 

 exposures of different lengths to a standard light, or to different 

 known intensities of light, on the same plate on which a nega- 

 tive has to be taken, the photographic values of the light acting 

 to produce the densities on the different parts of the developed 

 image can be readily found. Indeed, by making only two 

 different exposures to the same light, or two exposures to two 

 different intensities of light, and applying the law of density 

 of deposit in regard to them, a curve is readily made from 

 which the intensities of light necessary to give the different 

 densities of deposit in the image impressed on the same plate 

 can be read off. The application of such scales of density to 

 astronomical photographs, for example, cannot but be of the 

 highest interest, and will render the records so made many 

 times more valuable than they have hitherto been. I am in- 

 formed that the United States astronomers have already adopted 

 the use of such scales, which for the last three yeai-s I have 

 advocated, and it may be expected that we shall have results 

 from such scaled photographs which will give us information 

 which would before have been scarcely hoped for. 



One word as to a problem which we may say is as yet only 

 qualitatively and not quantitatively solved. I I'efer to the inter- 

 changeability of length of exposure for intensity of light. Put it 

 in this way. Suppose that with a strong light, L, a short ex- 

 posure, E, be given, a chemical change, C, is obtained : will the 

 same change, C, be obtained if the time is only an «th of the 

 light L, but n times the exposure ? Now this is a very important 

 point, more particularly when the body acted upon is fairly stable, 

 as, for instance, some of tlie water-colour pigments, which are 

 known to fade in sunshine, but might not be supposed to do so in 

 the light of an ordinary room, even with prolonged exposure. 

 Many experiments have been made at South Kensington as re- 

 gards this, more especially with the salts of silver, and it is 

 found that, for any ordinary light, intensity and exposure are in- 

 terchangeable, but that when the intensity of light is very feeble, 

 say the TxririiroTr of ordinary daylight, the exposure has to be 

 rather more prolonged than it should be, supposing the exact 

 interchangeability always held good ; but it has never been found 

 that a light was so feeble that no action could take place. Of 

 course it must be borne in mind that the stability of the sub- 

 stance acted upon may have some effect ; but the same results 

 were obtained with matter which is vastly more stable than the 

 ordinary silver salts. It may be said in truth that almost all 

 matter which is not elemental is, in time and to some degree, 

 acted upon by light. 



I should like to have said something regarding the action of 

 light on the iron and chromium salts, and so introduced the sub- 

 ject of platinotype and carbon printing, the former of which is 

 creating a revolution in the production of artistic prints. I have, 

 however, refrained from so doing, as I felt that the President of 



