Gradation of a Developed Photographic Image. 321 



of greys, and a photograph taken of it through red glass, which 

 practically cuts off all spectral rays except the red, and also through 

 blue glass, the gradation of greys in the negative would be much 

 more pronounced in the case of the red image than that of the blue, 

 and we come to the conclusion that for three-colour photographic 

 printing from a "red," a "green," and a "blue" negative this difference 

 should be a source of difficulty, and this is certainly the case. 



What scientific explanation there is of this difference in true 

 gradation factor is hard to say. It almost appears that in the case 

 of the blue waves acting on the atoms of the molecule of sensitive 

 salt, whilst the amplitude is increased the rate of oscillation is slightly 

 altered, gradually making the periodic motion of the waves of light 

 out of tune with the motions of the atoms ; whilst with the red rays, 

 which are vastly out of synchronism with the atomic swings, the 

 atoms got more nearly synchronous with them, and thus produce 

 more photographic action. In my work on ' The Action of Light in 

 Photography,' I have given a possible explanation of the difference 

 in effect caused by a feeble intensity and a great intensity of light, 

 and it may be that the same kind of explanation might hold good in 

 this newly found phase of the action of light. It appears that these 

 photographic phenomena are worthy of attention from the point of 

 view of molecular physics. 



It may be thought that these results might be peculiar to the salt 

 of silver experimented with. A further series of experiments were 

 conducted with the chloride of silver in gelatine. The maximum 

 sensitiveness of these plates was found to be near H in the solar 

 spectrum. The gradation was found to be least at this point, and 

 increased when rays on each side of this point were employed to act 

 on the film. In the blue near the F line, where the sensitiveness of 

 the plate was very small, the gradation was excessively steep, as it 

 also was in the extreme ultra-violet. 



JFave-lengths for Prismatic Sccde. 

 The following table shows the wave-lengths of the scale Nos. : — 



Scale ~No. 



X. 



Scale No. 



A. 



60 



673 



28 



478 



58 



652 



24 



464 



56 



633 



20 



452 



54 



615 



16 



440 



52 



600 



12 



430 



50 



585 



8 



420 



48 



572 



4 



410 



44 



548 







400 



40 



527 



-10 



381 



36 



508 



-20 



364 



32 



492 







