436 Scientific Proceedings^ Royal Dublin Society. 



film, may be rendered effective by tinging the film with a dye which absorbs 

 those wave frequencies. Here the electrons of the dye- molecules displaced 

 by the absorbed light are regarded as the cause of the ionization necessary to 

 form the photographic image on developmejit. 



This view would receive additional support if it could be shown that 

 sensitizers are effective at temperatures at which chemical actions do not 

 take place. 



Sensitization at Loiv Tenvgeratures. 



To test this point experiments were made on the following lines. ]lford 

 process plates, which are comparatively insensitive to red light, were cut up 

 into a number of small plates, 1 cm. x 2 em. Half of each of these was 

 sensitized by immersing it for a few seconds in a solution of " Sensitol 

 Ked" (1 : 76,000), as supplied by Ilford, Ltd., and made up according to the 

 maker's directions. After the plates liad dried, one was attached to a disc of 

 blackened cardboard (the " back "), 2 em. in diameter, by a rubber band 

 passing lengthwise across the plate, and at right angles to the line of junction 

 between the sensitized and untreated portions of the plate. The back was 

 then fixed at one end of a blackened cardboard tube, about 30 cm. long, the 

 other end of which was closed with a transparent Wratten light filter. No. 29, 

 transmitting only red light of wave-lengths greater than 615^/i. The card- 

 board tube, which formed a long camera, was set upright in a flask-shaped 

 Dewar vessel, so that the lower end containing the back was submerged in 

 the liquid air held in the vessel. The upper end projected from the neck. 

 The liquid air in the vessel, entering by perforations in the lower part of the 

 tubular camera, covered and completely submerged the plate. The Dewar 

 vessel and camera were set directly under a 100-watt ''■ gas-filled " lamp. The 

 plate was about 40 cm. distant from the lamp. All these arrangements and 

 operations were performed in total darkness. Exposure was made by 

 switching on the lamp. Exposure for one hour under these conditions 

 produced a latent image, which on development with rodinal showed only a 

 slight silver deposit on the untreated portion of the plate. A clear strip — 

 the shadow of the rubber band — without silver deposit crossed this portion. 

 The rest of the plate which had been sensitized showed out dark in contrast ; 

 but it, too, was crossed by the clear shadow. 



This experiment, which was frequently repeated, shows conclusively that 

 the sensitizer is effective at the temperature of liquid air (-185°C), and 

 hence supports the electronic theory of sensitization. There is, however, a 

 marked loss of sensitiveness at this temperature, for a control plate with 

 precisely the same arrangements, save that the flask did not contain liquid 



