Sept. 12, 1889] 



NATURE 



471 



so prolonged that its effect is visible, but also when the 

 exposure or intensity is so small that the effect is invisible and 

 only to he found by development. The reason for this 

 absorbent is not far to seek. If, for instance, silver chloride be 

 exposed to light in vacuo, although the chlorine atoms may be 

 swung off from the original molecule, yet they may only be 

 swung off to a neighbouring molecule which has lost one of 

 its chlorine atoms, and an interchange of atoms merely takes 

 place. If, however, a chlorine absorbent be present which has 

 a greater affinity for chlorine than has the silver chloride which 

 has lost one of its atoms,- then we may consider that the chlorine 

 atoms will be on the average more absorbed by the absorbent 

 than by the subchloride molecules. The distribution of the 

 swung-off atoms between the absorbent and the subchloride will 

 doubtless be directly proportional to their respective affinities 

 for chlorine, and so for the other salts of silver. If this be so, 

 then it will be seen that the greater the affinity of the absorbent 

 for the halogen the more rapid will be the decomposition of 

 the silver salt. This, then, points to the fact that if any increase 

 in the sensitiveness of a silver salt is desired it will probably be 

 brought about by mixing with it some stronger halogen absorbent 

 than has yet been done. 



The question as to what is the exact product of the decom- 

 position of a silver salt by the action of light is one which has 

 not as yet been fully answered. For my own part, I have my 

 strong beliefs and my disbeliefs. I fully believe the first action 

 of light to be a very simple one, though this simple action is 

 masked by other actions taking place, due to the surroundings 

 in which it takes place. The elimination of one atom from a 

 molecule of a silver salt leaves the molecule in an unsatisfied 

 condition, and capable of taking up some fresh atom. It is this 

 capacity which seemingly shrouds the first action of light, since 

 when exposure is prolonged the molecules take up atoms of 

 oxygen from the air or from the moisture in it. Carey Lea, of 

 Philadelphia, has within the last three years given some interest- 

 ing experiments on the composition of what he calls the photo- 

 chloride of silver, which is the chloride coloured by light, and 

 Prof. Hodgkinson has also taken up the matter. The con- 

 clusions the former has drawn are, to my mind, scarcely yet 

 to be accepted. According to the latter experimentalist the 

 action of light on silver chloride is to form an oxidized subsalt. 

 This can hardly be the case, except under certain conditions, 

 since a coloured compound is obtained when the silver chloride 

 is exposed in a liquid in which there is no oxygen present. 



This coloration by light of the chloride of silver naturally 

 leads our thoughts to the subject of photography in natural 

 colours. The question is often asked when photography in natural 

 colours will be discovered. Photography in natural colours not 

 only has been discovered, but pictures in natural colours have 

 been produced. I am not alluding to the pictures produced by 

 manual work, and which have from time to time been foisted 

 on a credulous public as being produced by the action of light 

 itself, much to the damage of photography and usually of the 

 so-called inventors. Roughly speaking, the method of pro- 

 ducing the spectrum in its natural colours is to chlorinize 

 a silver plate, expose it to white light till it assumes a violet 

 colour, heat till it becomes rather ruddy, and expose it to 

 a bright spectrum. The spectrum colours are then impressed in 

 their natural tints. Experiment has shown that these colours 

 are due to an oxidized product being formed at the red end of 

 the spectrum and a reduced product at the violet end. Photo- 

 graphy in natural colours, however, is only interesting from a 

 scientific point of view, and, so far as I can see, can never have 

 a commercial value. A process to be useful must be one by 

 which reproductions are quickly made ; in other words, it must be 

 a developing and not a printing process, and it must be taken in 

 the camera, for any printing process requires not only a bright 

 light but also a prolonged exposure. Now it can be conceived 

 that in a substance which absorbs all the visible spectrum the 

 molecules can be so shaken and sifted by the different rays that 

 eventually they sort themselves into masses which reflect the 

 particular rays by which they are shaken ; but it is almost— 

 I might say, quite — impossible to believe that when this sifting 

 has only been commenced, as it would be in the short exposure 

 to which a camera picture is submitted, the substance deposited 

 to build up the image by purely chemical means would be so 

 obliging as to deposit in that the particular size of particle 

 which should give to the image the colour of the nucleus on which 

 it was depositing. I am aware that in the early days of photo- 

 graphy we heard a good deal about curious results that had been 



obtained in negatives, where red brick houses were shown as 

 red and the blue sky as bluish. The cause of these few coin- 

 cidences is not hard to explain, and would be exactly the same 

 as when the red brick houses were shown as bluish and the sky 

 as red in a negative. The records of the production of the latter 

 negatives are naturally not abundant, since they would not attract 

 much attention. I may repeat, then, that photography in natural 

 colours by a printing-out process — by which I mean by the 

 action of light alone — is not only possible but has been done, 

 but that the production of a negative in natural colours from 

 which prints in natural colours might be produced appears, in 

 the present state of our knowledge, to be impossible. Supposing 

 it were not impracticable, it would be unsatisfactory, as the 

 light with which the picture was impressed would be very 

 different from that in which it would be viewed. Artists are fully 

 aware of this difficulty in painting, and take their precautions 

 against it. 



The nearest approach to success in producing coloured 

 pictures by light alone is the method of taking three negatives 

 of the same subject through different-coloured glasses, comple- 

 mentary to the three colour-sensations which together give to 

 the eye the sensations of white light. The method is open to 

 objection on account of the impure colour of the glasses used. 

 If a device could be adopted whereby only those three parts of 

 the spectnim could be severally used which form the colour- 

 sensations, the method would be more perfect than it is at 

 present. Even then perfection could not be attained, owing to 

 a defect which is inherent in photography, and which cannot be 

 eliminated. This defect is the imperfect representation of grada- 

 tion of tone. For instance, if we have a strip graduated from 

 what we call black to white (it must be recollected that no tone 

 can scientifically be called black, and none white) and photo- 

 graph it, we shall find that in a print from the negative the 

 darkness which is supposed to represent a grey of equal mixtures 

 of black and white by no means does so unless the black is not 

 as black nor the white as white as the original. The cause of 

 this untruthfulness in photography has occupied my attention 

 for several years, and it has been nay endeavour to find out 

 some law which will give us the density of a silver deposit on a 

 negative corresponding with the intensity of the light acting. 

 I am glad to say that at the beginning of this year a law dis- 

 closed itself, and I find that the transparency of a silver deposit 

 caused by development can be put into the form of the law of 

 error. 



This law can be scarcely empiric, though at first sight it appears 

 that the manipulations in photography are so loose that it should 

 be so. It is this very looseness, however, which shows that the 

 law is applicable, since in all cases I have tried it is obeyed. 

 That there are theoretical difficulties cannot be denied, but it is 

 believed that strictly theoretical reasoning will eventually reconcile 

 theory with observation. 



This want of truth in photography in rendering gradation, then, 

 puts it out of the range of possibility that photography in natural 

 colours can ever be exact, or that the three negatives system can 

 ever get over the difficulty. 



One of the reproaches that in early days was cast at 

 photography was its inability to render colour in its proper 

 monochromatic luminosity. Thus whilst a dark blue was 

 rendered as white in a print — that is, gave a dense de- 

 posit in a negative — bright yellow was rendered as black in 

 a print, or nearly so — that is, as transparent or nearly trans- 

 parent glass in the negative. ' To the eye the yellow might be far 

 more luminous than the blue, but the luminosity was in the 

 photograph reversed. I need scarcely say that the reason of this 

 want of truth in the photograph is due to the want of sensitiveness 

 of the ordinarily used silver salts to the least refrangible end 

 of the spectrum. Some fifteen years ago Dr. H. W. Vogel an- 

 nounced the fact that when silver salts were stained with certain 

 dyes they became sensitive to the colour of the spectrum, which the 

 dyes absorbed. This at once opened up possibilities, which, how- 

 ever, were not at once realized, owing perhaps to the length 

 of exposure required when the collodion process was employed. 

 Shortly after the gelatine process was perfected, the same dyes 

 were applied to plates prepared by this method, which, although 

 they contained the same silver salts as the old collodion process, 

 yeiper se were very much more sensitive. A new era then dawned 

 for what has been termed isochromatic and orthochromatic 

 photography. The dyes principally used are those belonging to 

 the eosin group and cyanine — not the ordinary cyanine dye of 

 commerce, but that discovered by Greville Williams. For a dye 



