ently attacked by the developer. The action is not altogether independent of the effect described below.
(3) Method depending on the variation of electric potential, by which an E. M. F. is produced between the acted and unacted portions of a substance originally iso-electric.
(4) Method depending on the conductivity variation produced by the strain.
In the following investigations I shall employ specially the two last methods, and hope to demonstrate the fundamental unity of effects of visible and invisible radiation on matter. The subject is very extensive, and I propose to deal with it, as briefly as is compatible with clearness, in the three accompanying papers:—
I. "On the Continuity of Effect of Light and Electric Radiation on Matter." In this paper various experimental methods and results will be given, which can only be explained on the supposition that the observed effects are due to strain.
II. "On the Similarities between Mechanical and Radiation Strains." If the effects as described above are really due to strain, then similar results might be brought about by artificially producing strain in regard to which there can be no possibility of doubt, such as strain produced by mechanical means.
III. "On the Strain Theory of Photographic Action." Having shown the strain effect due to light, I will show how some of the obscure phenomena in photography receive a simple explanation on the above theory.
