192 



Prof. J. C. Bose. 



[Apr. 18, 



parably higher than any that can be produced by a spark from an 

 electric machine. Mr. E. W. Wood* obtained reversal with a single 

 spark, when the photographic lens was wide open, but there was no 

 reversal with four sparks, the lens aperture being reduced to one- 

 fourth. The quantity of light was the same in the two cases, but 

 the time-rate of illumination was different. This curious result 

 would no longer appear anomalous, if we bear in mind the experiment 

 in which the influence of time-rate was shown. 



(2.) In trying to obtain photographs by heat radiation on sensitised 

 papers coated with a mixture of silver and mercury iodides, the 

 following curious effect was observed. The sensitised paper was 

 exposed to heat radiation and became uniformly reddish in colour. 

 A mask with cut-out letters was now put on it, and the sensitised 

 paper was allowed to cool. The rate of cooling was very rapid at 

 the places exposed by the cut-out letters, whereas at the covered 

 portions the rate of cooling was very much less. After a long time 

 when the sensitive paper had cooled down to a uniform temperature, 

 prints were still visible, the effect being evidently due to the different 

 rates of emission in the screened and unscreened parts. 



(3.) Major-General "Waterhouse in his paperf mentions an anomalous 

 case which seems to be explicable from considerations given above. 

 He took a polished silvered glass plate, and put it into a printing 

 frame with a cut-out paper mask and mica screen in which were cut- 

 out initials, just as if it were going to be exposed to the sun ; but 

 instead of exposure to light the plate was gently warmed for about 

 5 minutes over a spirit lamp, and then developed with mercury. The 

 cut-out initials came out distinctly in dark lines. It seems to me that 

 in this experiment, as the plate was uniformly warmed, the difference 

 between the screened and unscreened portions could only be in the 

 different rates of emission. 



9. Phenomenon of Recurrent Reversals. 



The fourth stage in the curve for the action of light (see fig. 16) 

 will be found specially interesting with reference to photographic 

 reversals. These reversals are found to be recurrent. Thus, starting 

 with a neutral condition, we obtain the first negative with a moderate 

 exposure ; longer exposure will tend to reduce the intensity of the 

 negative and give rise to a neutral condition. Further exposure gives 

 rise to a positive, then a second neutral, and again a succeeding negative 

 stage, and this often goes on in recurrent series. 



Such recurrent reversals are also exhibited (see fig. 18) by a sub- 

 stance under continuous mechanical vibration. In my paper on " Elec- 



* < Nature,' November 30, 1899. 

 f Loc. cit. 



