76 M. C. Lea on the Theory of Photo-chemistry. 
The general views here expressed will be found to throw light 
upon other obscure photo-chemical phenomena besides the latent 
physical image. Of these I shall briefly cite one. 
If light fall upon a body decomposable by light, its energy 
will be expended in two directions, Part will be transmitted to 
surrounding objects, part will be expended in dissociating one 
or more of the elements of the body, As the intensity of the 
light increases, the amplitude of the excursions also increases, 
and a larger proportion will be expended in decomposition. It 
therefore follows that the decomposition effected will not be in 
the ratio of the intensity of the light, but will be greater ina 
strong light and less in a weak; that is, that a light of half 
strength acting for a double time, will not effect an equal amount 
f mposition. Now this exactly accords with the univer- 
sal experience of photographers who find that where media of 
unequal thickness are interposed between a sensitive surface and 
the light, as in the case of a negative superimposed upon chlo- 
rid of silver, the weaker the light the greater will be the con 
trast in the degrees of effect produced upon the different parts 
of the film; so much so that this is systematically adopted as @ 
base of operation. 
To enlarge farther here on the application of these principles, 
and the light they throw upon photographic phenomena, would, 
however, be inconsistent with the limits I have imposed upon 
myself. My object here has been to prove the existence of a 
CE but hitherto overlooked property of matter, 
further to show the identity of this property with what in pho- 
to- istry has been vaguely described as the physical image. 
This explanation of these obscure phenomena seems so simple 
and sufficient as scarcely to require proof. Nevertheless that 
proof is afforded by the reactions of iodid of silver, perhaps the 
most remarkable substance with which chemistry makes us ac- 
quainted. This theory rests upon two properties for whose eX- 
istenee I have long contended, and which I believe I have suc- 
pees a in establishing, namely, the sensitiveness to light of — 
even when perfectly rsolated, and its spontaneous recovery 
that sensitiveness after obliteration through powerful action of 
light, by simply remaining in darkness. From these facts I 
deduce the conclusion that the latent image is simply due to & 
phosphorescence of chemical or actinic rays to which property I 
of actinescence. 
give the name 
. > 
