Section II.--_The Photographic Properties of the Salts of Silver._--The action of Light upon Nitrate of Silver--upon Chloride, Bromide, and Iodide of Silver.--The blackening of Chloride of Silver explained.--Simple experiments with sensitive Photographic Paper 18 CHAPTER III. ON THE DEVELOPMENT OF AN INVISIBLE IMAGE. Simple experiments illustrating the process 25 Section I.--_Chemistry of the Substances, employed as Developers._--Development shown to be a process of reduction.--The chemistry of the principal reducing agents, Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron 26 Section II.--_The Reduction of Salts of Silver by Developing Agents._--Reduction of Oxide of Silver--of Nitrate and Acetate of Silver.--Varied appearance of Metallic Silver when finely divided.--The reduction of the Iodide and other Salts of Silver containing no Oxygen 30 Section III.--_Hypothesis on the Formation and Development of the Latent Image._--Explanation of the terms under- and over-exposure.--Diagram of Molecular change.--Moser's experiments on development.--Curious perversions of development 34 CHAPTER IV. ON "FIXING" the PHOTOGRAPHIC IMAGE. Chemistry of the various substances which may be employed as Fixing Agents.--Ammonia, Alkaline Chlorides, Bromides, and Iodides.--Hyposulphite of Soda.--Cyanide of Potassium 41 CHAPTER V. ON THE NATURE AND PROPERTIES OF LIGHT. Section I.--_The compound Nature of Light._--Its decomposition into elementary coloured rays.--Division of these rays into Luminous, Heat-producing, and Chemical Rays 46 Section II.--_The Refraction of Light._--Phenomena of simple refraction by parallel and inclined surfaces.--Refraction from curved surfaces.--The various forms of Lenses.--The Foci of Lenses.--Formation of a Luminous Image by a Lens 49 Section III.--_The Photographic Camera._--Its simplest form.--The field of the Camera.--Chromatic aberration.--Spherical aberration.--The use of Stops.--The double, or Portrait combination of Lenses.--Variation between the Visual and Chemical Foci in Lenses 54 Section IV.--_The Photographic Action of Coloured Light._--Diagram of Chemical Spectrum.--Illustrative experiments.--Superior sensibility of Bromide of Silver to coloured light.--Mode in which dark-coloured objects are Photographed 60 Section V.--_On Binocular Vision and the Stereoscope._--Phenomena of Binocular Vision.--Theory of the Stereoscope.--Wheatstone's reflecting Stereoscope.--Brewster's Stereoscope.--Rules for taking Stereoscopic pictures 66 CHAPTER VI. THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION.