TRANSACTIONS OF SECTION A. 485 



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 my 

 endeavour to tind 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 disclosed itself, and I iind that the 

 transparency of a silver deposit cause 1 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 dilKculties 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 dilHculty. 



One of the reproaches that in early days was cast at photography was its in- 

 ability to render colour in its proper monochromatic luminositj-. Thus whilst a 

 dark blue was rendered as white in a print — that is, gave a dense deposit in a nega- 

 tive — bright yellow was rendered as blaclc in a print or nearly so — that is, as 

 transparent or nearly transparent 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 tlie 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 announced 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, however, were not 

 at once realised, 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, yet j^cr se were very 

 much more sensitive. A new era then dawned for what has been termed iso- 

 chromatic and orthochromatic photography. The dyes principally used are those 

 belonging to the eosin group and cyanin— not the ordinary cyanine dye of com- 

 merce, but that discovered by Greville "Williams. For a dye to be of use in this 

 manner it may be takfii as an axiom — first propounded by the speaker, it is believed 

 — that it must be fugitive, 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 con- 

 ducted, 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 



