THEORETICAL. 17 was found to be the alkali which those substances contained. We must further mention Narcotine and Codeine as good sensitizers, and, most important of all, silver nitrate and certain aniline dyes. Silver nitrate is a bromine-absorber and this chemical sensitizer, as well as other halogen absorbers, is of great importance. The sensitizers now generally in use are formed by certain aniline dyes and their compounds with silver nitrate, and isochromatic photography is indebted to Dr. E. Albert, of Munich, who was the first to prepare colour-sensitive collodion emulsions of high rapidity, by introducing eoside of silver into the emulsion. The organic sensitizers are only of value in collodion emulsions which are to be used for transparency, opal, and lantern slide work, whereas the chemical and the optical or colour sensitizers are chiefly made use of in process work, and for three-colour portrait and landscape work where ortho chromatism and great rapidity are requisite. 3.—SENSITIZING OF COLLODION EMULSION FOR ISOLATED PARTS OF THE SPECTRUM. Collodion emulsion can be sensitized for nearly all colours of the spectrum, without losing its general sensi tiveness, which as a rule is greatly increased by the addition of aniline dyes. The colour-sensitizing of collodion emulsion can be accomplished with greater accuracy than with gelatine dry plates, a phenomenon which has been investigated by Baron Hiibl, who stated that the screening defect of gelatine plates, caused by the quick absorption of a dye, is so great that a complete shifting of the region of sensi tiveness takes place, which makes the accurate adjust ment of the colour-sensitiveness of a dry plate an extremely difficult and uncertain process. Collodion emulsion is almost free from this defect, the film does not stain very