Helio Process.--An Entire Process for Producing Collodion Positives and Negatives with one Bath, and in much less time than by any other known Process: by Helio--Photographic Patents 164 CHAPTER VIII. The Collodio-Albumen Process in Detail 190 CHAPTER IX. On a Mode of Printing Enlarged and Reduced Positives, Transparencies, &c., from Collodion Negatives--On the Use of Alcohol for Sensitizing Paper--Recovery of Silver from Waste Solutions,--from the Black Deposit of Hypo Baths, &c.--The Salting and Albumenizing Paper--On the Use of Test Papers--Comparison of British and French Weights and Measures 191 CHAPTER I. LIGHT--SOLAR SPECTRUM--DECOMPOSITION OF LIGHT--LIGHT, HEAT AND ACTINISM--BLUE PAPER AND COLOR FOR THE WALLS OF THE OPERATING ROOM--PROPORTIONS OF LIGHT, HEAT AND ACTINISM, COMPOSING A SUN-BEAM--REFRACTION--REFLECTION--LENSES--COPYING--SPHERICAL ABERRATION--CHROMATIC ABERRATION. It has been well observed by an able writer, that it is Impossible to trace the path of a sunbeam through our atmosphere without feeling a desire to know its nature, by what power it traverses the immensity of space, and the Various modifications it undergoes at the surfaces and the interior of terrestrial substances. Light is white and colorless, as long as it does not come in contact with matter. When in apposition with anybody it suffers variable degrees of decomposition, resulting in color, as, by reflection, dispersion, refraction and unequal absorption. To Sir I. Newton the world is indebted for proving the compound nature of a ray of white light emitted from the sun. The object of this work is not to engage in an extended theory upon the subject of light, but to recur only to some points-of more particular interest to the photographic operator. [Sidenote: PRISM, SOLAR SPECTRUM.] The decomposition of a beam of light can be noticed by exposing it to a prism. If, in a dark room, a beam of light be admitted through a small hole in a shutter 7 it will form a white round spot upon the place where it falls. If a triangular prism of glass be placed on the inside of the dark room, so that the beam of light falls upon it, it no longer has the same direction, nor does it form a round spot, but an oblong painted image of seven colors--red, orange, yellow, green, blue, indigo, and violet. This is called the solar spectrum, and will be readily understood by reference to the accompanying diagram, Fig. 1. [Illustration: Fig. 1.]