662 REPORT — 1895. 



as observed in dyed fabrics, and the experimental method that he used is open to 

 criticism. 



The author's observations on the fading of the various sensitisers, when exposed 

 to light in contact with gelatin, lead him to the conclusion that, although all the 

 sensitisers are readily affected by light, the order of sensitising effect does not 

 necessarily correspond with the order of fading, whether the dj"es belong to the 

 same chemical group or not. 



There are two chief hypotheses as to the mode in which the dyes act, namely : 

 (1) the view held by Abney that the dye itself is oxidised by the action of light, 

 the oxidation product remaining in contact with the silver bromide ; and when the 

 plate is treated with the developer, the latter and the oxidation product acting 

 simultaneously on the silver bromide bring about its reduction ; and (2) the view 

 first definitely formulated by Eder, and endorsed by Vogel, namely, that the energy 

 absorbed by the dyed silver bromide is partially used up in bringing about the 

 chemical decomposition of the silver bromide, instead of being almost entirely con- 

 verted into heat, as when absorbed by the d^e alone. 



The author has found that the less refrangible rays will produce a photographic 

 image on the sensitised gelatino-bromide plates, when they are immersed in 

 powerfully reducing solutions, such as a mixture of sodium sulphite and pyro- 

 gallol. This holds good for cyanin, the eosin dyes, the rhodamines, and quinoline 

 red, whether the seusitiser has been added to the emulsion or has been applied to 

 the prepared plate in the form of a bath. It is, therefore, impossible to attribute 

 the sensitising effect to any intermediate oxidation of the dye. 



Experiments with various reagents such as potassium bromide, potassium dichro- 

 mate, mercuric chloride, and dilute hydrogen peroxide seem to show that the 

 chemical nature of the latent image produced by the less refrangible rays on the 

 specially sensitised plates is precisely the same as that of the latent image produced 

 by the more refrangible rays in the ordinary way. 



Further proof in the same direction is aff'orded by the fact that the effect of the 

 sensitisers extends to the production of a visible effect by the prolonged action of 

 light. 



The balance of evidence is therefore greatly in favour of the view that the dye 

 absorbs the particular group of rays, and, in some way- which is not at all clear, 

 hands on the energv' to the silver bromide, with which it is intimately associated, 

 iind which is thereby decomposed. 



For the pi-esent, for want of a better word, the phenomenon might be distin- 

 guished a.s 2}ftoto-rataIt/sis, und the sensitiser might be described as & jihoto-catalytic 

 agent. As yet no connection can be traced between the chemical constitution and 

 the general physical properties of a dye, and its sensitising action. 



3. Report of the Committee on the Action of Light on Dyed Colours. 



See Reports, p. 263. 



4. On some Stilbene Derivatives. 

 By J. J. SUDBOROUGH, D.Sc, Ph.D., F.I.C. 



The author has prepared monochloro-, methyl-chloro-, and ethyl-chloro- 

 stilbene by the action of phosphorus pentachloride on deoxybenzoin and on its 

 methyl and ethyl derivatives. The monochloro-stilbene differs from that described 

 by Zinin (' Annalen,' 149. 375), as it is a solid, which crystallises from alcohol in 

 large colourless plates. It melts at 63°-54°, and fields additive compounds with 

 bromine, with chlorine and with ' nitrous acid.' These, together with the corre- 

 sponding compounds obtained from methyl- and from ethyl-chloro-stilbene, are 

 described. An oily monochloro-stilbene, corresponding to that of Zinin, has also 

 been prepared, and is being subjected to further examination in order to determine 

 whether it is merely an impure form of the crystalline compound or a true stereo- 

 isomeride. 



