( 738 ) 
of ABC....//OX, or from O onward recedes in positive direction 
from the abscissa, then it will never be possible to obtain by varying 
(D,+ D,) the colour of one or two of the subhaloids, but the photo- 
chemical decomposition will always progress more or less rapidly 
in the same way in all cases. However, the results of the experiments 
mentioned below cannot be reconciled to this supposition. 
The influence of the size of the grain. As the photo-chemical 
decomposition of silver haloids is restricted to the surface of the grains, 
it is easy to understand that the size of the grain must influence the 
decomposition process. The finer the grain, the more “saturated” 
the colour, the coarser the grain, the weaker the colour will be. 
As in formula (III) D becomes greater owing to the finer grain, 
D, and D, will therefore also increase. Consequently by exposing 
coarser and finer silver haloid grains emulsionized under as mneh 
as possible the same circumstances, it can be determined experimen- 
tally, to what extent D changes through an increase of D,, and an 
increase of (D,+D,) depending upon it. I have made this experiment 
with a fine-grain silver chloride gelatine emulsion, and silver chloride 
gel emulsionized in gelatine, but found, except in the saturation of 
the colours, a very small difference, which might even be put down 
to an inaccuracy in the experiment, viz. a slight difference in the 
amount of chemical sensitizer. 
Jontrol. If in formula (III) D, is made smaller, D will become 
greater, and the colour of a subhaloid richer in halogen will appear. 
The same thing may, however, be obtained in another way. The 
equilibrium pressure is smaller with a weaker light intensity (approaches 
zero in the dark), but in this case D, will also decrease a well 
as the term (D,+D,) dependent upon it, and, as has been shown 
by the enquiry into the influence of the size of the grain upon silver 
haloid, D thereby changes little or nothing. The consequence of 
making the photo-chemical decomposition take place with a smaller 
light intensity, will therefore consist in the appearance of subhaloids 
richer in hatogen. Therefore, by lowering D as well as the light 
intensity, the same subhaloids richer in halogen, in the opposite case 
the same subhaloids poorer in halogen, will be seen to appear. 
Another means of control is afforded by increasing D after the 
photo-chemical decomposition has been interrupted by introducing 
halogen into the preparation; in this case the same subhaloids richer 
in halogen must be obtained back again. This method, however, has 
the drawback, that owing to the absorption spectrum of the halogen, 
the colours of the subhaloids are to be observed in a less pure 
condition. 
