LIST OF PLATES. I. Three-Colour Reproduction from a Water-Colour Drawing - Frontispiece II. Comparative Speed Tests - - • 14-15 III. Absorption of Dyes .... 22-23 IV. Tallent’s Spectroscopic Camera - - 26-27 V. Monochrome Representation of the Colour Scale 52-53 VI. Photo-Micrographs of Metzograph Screen Effects .... 64-65 VII. Markings on Collodion Emulsion Plates 80-81 VIII. Faults in Collodion Emulsion Nega tives 80-81 IX. Effects of Irradiation - - - 80-81