224 Scientific Proceedings, Royal Dublin Society. 
affecting the dyed plate the more seriously, as this is prepared to 
use the long wave-lengths through the intervention or aid of the 
dye stuff. The experiments, in fact, revealed that, save for the 
survival of some activity along the special dense band on the 
green, the ortho-plate was virtually reduced by cold to the limits 
of sensitiveness of the undyed plate. 
While I am inclined to think that the silver bromide molecule 
is in some degree affected as above, I do not think the experiments 
on the gelatino-bromide plate prove that it alone is affected in 
the orthochromatic plate. On the view (favoured by Abney) that 
the sensitizing action of the dye is a chemical one, started into 
operation by a photo-chemical change in the dye, it appears to me 
very probable that the experiments are, in fact, only one more 
example of the already long series of chemical actions known to 
be dependent upon temperature for their activity. Possibly, too, 
the initial photo-chemical action upon the dye is reduced in inten- 
sity at low temperatures. In short, the events set in operation by 
the light waves are complex, and all concerned with molecular 
stability, whether towards the periodic forces of light or their 
mutual attractions. It is most probable that molecular stability 
towards either action is dependent on the quantity of energy 
possessed by the molecular system. 
In the experiments described above, care must be taken that 
the plates have arrived at uniform temperature before develop- 
ment, or quite other effects would arise. With my own arrange- 
ments I am, of course, unable to estimate the temperature of the 
film. It is certainly not nearly so low as that reached by the mixed 
carbon dioxide and ether (—81°). With special arrangements, 
probably, even more marked results could be obtained. Photo- 
graphs of yellow flowers upon cooled isochromatic plates contrasted 
with photographs taken on plates at air temperature would show 
the effects of cold, no doubt, strikingly. In very cold climates 
isochromatic plates will possess but little advantage over ordinary 
plates, except they be maintained at a sufficiently high tempera- 
ture when being exposed. However, the influence of length of 
exposure has still to be investigated. It might be that long 
exposure through a medium opaque to the blue and violet light 
would restore the ‘iso ’-chromatic quality. 
