Solution of Cyanide of Potassium is a most energetic agent in dissolving the insoluble Silver Salts: far more so, in proportion to the quantity used, than the Hyposulphite of Soda. The Salts are in all cases converted into Cyanides, and exist in the solution in the form of soluble double Salts, which, unlike the double Iodides, are not affected by dilution with water. Cyanide of Potassium is unadapted for fixing positive proofs upon Chloride of Silver; and even when a developer has been used, unless the solution is tolerably dilute, it is apt to attack the image and dissolve it.
[CHAPTER V.]
ON THE NATURE AND PROPERTIES OF LIGHT.
The present Chapter is devoted to a discussion of the more remarkable properties of Light; the object being to select certain prominent points, and to state them as clearly as possible, referring, for information of a more complete kind, to acknowledged works on the subject of Optics.
The Chapter will be divided into five Sections:—first, the compound nature of Light; second, the laws of refraction of Light; third, the construction of Lenses and of the Camera; fourth, the Photographic action of coloured Light; fifth, on Binocular Vision and the Stereoscope.
SECTION I.
The Compound Nature of Light.
The ideas entertained on the subject of Light, before the time of Sir Isaac Newton, were vague and unsatisfactory. It was shown by that eminent philosopher, that a ray of sunlight was not homogeneous, as had been supposed, but consisted of several rays of vivid colours, united and intermingled.