proceedings: philosophical society 525 



In this work the radiation stimuH of different wave lengths were of 

 equal energy value. Hence, if this phenomenon were the result of 

 heating and of electrolytic action, the photo-positive response 

 should occur in the short wave lengths where the absorption is the 

 greatest in the cr^'stal, and the photo-negative response should occur 

 in the long wave lengths, where the photo-electric reaction is the greatest. 

 In this respect, the observed phenomena are just the reverse of what 

 one would expect. 



Discussion: This paper was illustrated by lantern slides and was 

 discussed by Messrs. Sosman and Hawkesworth. 



S. M. Burka: Hyper sensitizing commercial panchromatic plates. 



Ordinary photographic plates, which owe their sensitivity to the 

 silver halides alone, are sensitive only to the violet and blue regions of 

 the spectrum. If, however, suitable dye-stuffs be added, either by the 

 laboratory method of bathing the plates in a solution of the dye, or 

 by the commercial method of incorporating the dye in the emulsion, 

 the plates become sensitive to other regions. 



The effect of ammonia on the sensitivity of commercial plates con- 

 taining dyes was investigated. As a result of numerous trials, using 

 varying concentrations of ammonia, the procedure finally adopted 

 was to bathe the plates before exposure for four minutes at 16° to 

 18° C. in a solution of 3 cc. ammonia (20 per cent NH3) in 75 cc. water 

 and 25 cc. ethyl alcohol and dry as rapidly as possible. They were then 

 exposed in a grating spectrograph and the spectral sensitivity deter- 

 mined. In a number of cases the speed was also determined by a 

 sensitometer of the Chapman Jones type, and by the Hurter and 

 Driffield method. 



The sensitivity of orthochromatic (sensitive to green and yellow) 

 plates showed, with one exception, no appreciable change. Every 

 panchromatic (sensitive to all colors) plate tried (nearly a dozen different 

 brands) had its speed increased at least 100 per cent to white light and 

 over 400 per cent to red. The sensitivity was also extended 100 or 

 more Angstrom units into the red. The strong minimum in sensitivity 

 in the green shown by most plates was to a great extent smoothed out. 

 The most striking increase in sensitivity was shown by a Wratten and 

 Wainright panchromatic plate which was used three years after its 

 "expiration" date. The speed was increased over 600 per cent through- 

 out the region from H^s to beyond Ha. 



The action of the ammonia seems to be specific towards the pinaverdol 

 and pinacyanol series of dyes. Practically all orthochromatic plates 

 are dyed with erythrosin. The exceptional plate mentioned above is a 

 pinaverdol plate. 



Bathing a plate after exposure but before development has no effect 

 on the speed. 



The ammoniated plates deteriorate rapidly, showing chemical fog 

 in development, so that most brands are useless four or five days after 

 treating. The plates should, therefore, be used a day or two after 



