484 BEPOBT— 1889. 



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

 Hodgkinson has also taken up the matter. The conclusions 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 oxidised sub- 

 salt. 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 to the so-called 

 inventors. Roughly speaking, the method of producing the spectrum in its natural 

 colours is to chlorinise 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 oxidised product being formed at the red 

 end of the spectrum and a reduced product at the violet end. Photography in 

 natural colours, however, is only interesting from a scientifi<! 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 de- 

 veloping 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 ])articular 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 ba 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 par- 

 ticle 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 photography 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 co- 

 incidences 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. 1 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 stiite of our knowledge, to be impossible. Supposing it were not im- 

 practicable, it would be unsatisfactory, as the light with which the picture was 

 impressed would be very difl'erent 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, complementary 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 spectrum 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 gradation 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 scien- 

 tifically be called black, and none white), and photograph it, we shall find that in 



