Page:Popular Science Monthly Volume 39.djvu/429

Rh sections of the book have been assigned to educators of recognized eminence and skill, especially qualified to deal with the particular topics which are especially given them. Thus, the sections on motion, energy, force, the properties and constitution of matter, solids, liquids, gases, and mechanics proper have been prepared by Prof. S. W. Holman, of the Massachusetts Institute of Technology; those on heat, light, frictional and voltaic electricity, by Prof. Francis E. Nipher, of Washington University, St. Louis; the chapter on sound, by Prof. Alfred M. Mayer, of Stevens Institute; and the sections relating to magnetism and the practical applications of electricity, by Prof. F. B. Crocker, of the Columbia College School of Mines. . Among the specific features claimed for the work are the thorough and original treatment of motion, energy, force, and work; and the modern and appliable conception of the nature, transformation, and conservation of energy, and of the relation between energy and force. The book is adapted to students fourteen years old and upward, but, by the omission of certain classes of paragraphs, it may be made comprehensible to younger learners. It has been the aim of the authors not to teach results merely, but to show how these results have been reached, and what practical use is made of them. Precedence is everywhere given to the practical.

photography can now claim a literature of its own, the historical aspect of the art has naturally been neglected. Especially in later years has discovery followed upon discovery so closely as to allow little time for retrospection. The rapid introduction of different processes has been followed by numerous treatises on special methods, and manuals on the general practice of the art abound, but the field of reminiscence has been mostly untrodden save by Mr. Werge, who in 1880 published an account of the origin and process of photography. The present volume easily divides itself into three sections: the first containing an outline of the development of photography; the second, a chronological record; and the third, personal recollections. The author marks four periods in the history of the art: the dark ages; the age of publicity; the epoch of collodion triumphant; the epoch of gelatin successful. The dark ages include the time from the thirteenth century to the advent of the daguerreotype. The first three centuries may be justly regarded as very nebulous indeed, without the glimmering of a photographic ray, and marked only by the discovery of the agents that were long afterward employed in producing pictures. Among such may be counted the invention and perfection of the camera obscura and the metallic researches of the early alchemists. The first step toward acquaintance with actinic influence was the observation of the darkening of chloride of silver in the sixteenth century. As chemical knowledge increased, other phenomena were noted; and finally Scheele, the Swedish chemist, experimented with the prism and demonstrated the greater activity of the violet ray. Meanwhile the double achromatic lens had been constructed, and the possibility of sun portraiture was realized. Scientific men essayed the problem, and in 1839 M. Daguerre's process was given to the world by the French Academy. In spite of this official announcement, there seems to be every reason to agree with Mr. Werge that England had preceded France in photographic discovery, as the Rev. J. B. Reade produced ineffaceable pictures upon paper by means of tannin and hyposulphite of soda in 1837.

The next important advance was made by Talbot in demonstrating the latent image to be the basis of photogenic manipulation. The subsequent discovery and solution of gun-cotton made possible the collodion nega. tives of Archer; and in 1850, a gelatin process was introduced by M. Poitevin. The art then enters upon its marvelous series of developments; the heliochromes of Niepce de St. Victor, photo-engraving on steel, orthochromatic plates, platinotypes, carbonprinting, and gelatin dry plates. As a dutiful daughter of Science, Photography assists in her researches, makes visible the stars, the mechanism of muscular movement, and the progress of disease.

The chronological record given by Mr. Werge contains not only a list of discoveries and inventions pertaining to photography,