Chapter Iv. Treats of the Fixing of Photographic Impressions, in Order to

render them indestructible by diffused light.

Chapter V. contains a sketch of the Optics of Photography--the decomposition of white Light into its elementary rays, the Photographic properties of the different colours, the refraction of Light, and construction of Lenses. In the last Section of the same Chapter will be found a short sketch of the history and use of the Stereoscope.