DIFFICULTY OF EXPLAINING REVERSAL.
It must be confessed that it is extremely difficult to attempt to explain, from either • a chemical or physical point of view, the various facts underlying reversal. It is very probable that the prolonged exposure necessary to produce solarisation is more photo-chemical in its behaviour than physical. It is only necessary to con sider the possibility of having on a plate, in less than microscopic quantities, unal tered silver haloid, reduced silver haloid (subhaloid or .oxylialoid, or a combina tion of the two), silver haloid under mole cular tension, gelatine partly oxidised, partly halogcnised, and partly under mole cular tension, to see how very complicated the subject becomes. In the present state of photo-chemical knowledge the so-called explanations are of a purely speculative character.
ExPERDIExt WITH MERCURIC CHLORIDE.
In connection with this subject of the destruction of the latent image, the fol lowing experiment of Reiss (" Chem. Zeit." 26 [10]) is interesting. lie utilised the
well-known destructive action of mercuric chloride on the invisible image to render exposed plates fit for a second exposure. The exposed plate, containing the image to be destroyed, is first treated with a solution of 5 per cent. mercuric chloride and then well washed. It is next quickly immersed in an amidol developer, which seems to facilitate the action of the light in the second exposure, dipped in water, and then exposed while wet. The ex posure takes from about 100 to 150 times that of the first, and a much longer de velopment, to produce the second latent image. It is rather curious that no fog results. The negatives obtained are well covered in the lights, but are perfectly clear in the shadows. Apparently the action of the mercuric chloride on the latent image induces a far greater change than that involved in merely converting it to its original condition.