Photographic printing methods : a practical guide to the professional and amateur worker

Front matter

The Scovill Photographic Series.

Price, Per Copy.

No. i.— THE PHOTOGRAPHIC AMATEUR.— By J. Traill Taylor. A Guide to the Young Photographer, either Professional or Amateur. (Second

Preface

I. General Remarks on Sensitive Surfaces, etc.

II. Preliminary Remarks on Exposure, Development, Fixing, etc.

III. Calotype.

IV. Sensitive Surfaces on Glass— Preparation of the Glass.

V. The Albumen Process.

VI. The Old Collodion Process, Wet Plates.

VII. The Collodion Process, Dry Plates.

VIII. Collodion Emulsion— Collodio-bromide of Silver.

IX. The Gelatine Process.

X. Coating the Plates.

XI. Development, Fixing, etc.

XII. Paper Negatives— Stripping Films on Paper, Card-board, and Collodion.

XIII. Failures in the Gelatino-bromide Process.

XIV. Methods of Stripping Films from Glass Plates.

XV. Color-sensitive Plates.

XVI. Black and White Negatives.

XVII. Instantaneous Photography.

XVIII. Touching-up the Negative.

XIX. Photo-micrography.

XX. Micro-photography.

XXI. The Transformation of Negatives into Positives.

XXII. Obernetter’s Method for the Direct Production of Negatives from Negatives. Index.

It contains a Meisenbach Frontispiece of a pictorial subject from a negative made by the author. Full description of his method for making the Emulsion ; also, much other valuable information never before pub¬ lished.

Profusely illustrated with cuts, two full-page pictorial Mosstypes, and more than two hundred pages of valuable reading matter.

A companion to “Photographic Printing Methods."

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iv

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EXPERIMENTAL SCIENCE.— By George M. Hopkins. 740 pages, 680 illustra¬ tions. By mail, post-paid . . 4 00

THE CHEMISTRY OF PHOTOGRAPHY.— By Prof. Raphael Meldola . 2 00

THE FERROTYPER’S GUIDE.— Cheap and complete. For the Ferrotyper, this

is the only standard work. Seventh thousand . 75

THE PHOTOGRAPHIC STUDIOS OF EUROPE.-By H. Baden Pritchard,

F.C.S. Cloth bound . 100

Paper cover . . . 50

ART OF MAKING PORTRAITS IN CRAYON ON SOLAR ENLARGE¬ MENTS. — (Third Edition.) By E. Long . . 1 00

PHOTOGRAPHY APPLIED TO SURVEYING.— Illustrated. By Lieut. Henry

A. Reed, U. S. A. Cloth bound . 2 50

HISTORY AND HAND BOOK OF PHOTOGRAPHY.— Translated from the

French of Gaston Tissandier, with seventy illustrations. Cloth bound . 75

A COMPLETE TREATISE ON SOLAR CRAYON PORTRAITS AND TRANSPARENT LIQUID WATER COLORS.-By J. A. Barhydt. Prac¬ tical ideas and directions given. Amateurs will learn ideas of color from this book that will be of value to them. And any one by carefully following the directions on Crayon will be able to make a good Crayon portrait . 50

ART RECREATIONS. — A guide to decorative art. Ladies’ popular guide in

home decorative work. Edited by Marion Kemble . 2 00

AMERICAN CARBON MANUAL. — For those who want to try the Carbon printing

process, this work gives the most detailed information. Cloth bound . 50

MANUAL DE FOTOGRAFIA. — By Augustus Le Plongeon. (Hand-Book for

Spanish Photographers.) . . 1 00

SECRETS OF THE DARK CHAMBER. -By D. D. T. Davie . 50

AMERICAN HAND-BOOK OF THE DAGUERREOTYPE. — By S. D. Humphrey. (Fifth Edition.) This book contains the various processes employed in taking Heliographic impressions . 25

THE PRACTICAL PHOTOGRAPHIC ALMANAC . 25

BRITISH JOURNAL ALMANAC FOR 1878, 1883, 1887 . 25

PHOTO NEWS YEAR BOOK OF PHOTOGRAPHY FOR 1870, 1871, 1887, 1888.. 25

THE PHOTOGRAPHER’S FRIEND ALMANAC . 25

AMERICAN ALMANAC OF PHOTOGRAPHY . 25

THE PHOTOGRAPHER’S BOOK OF PRACTICAL FORMULAS.— Compiled

by Dr. W. D. Holmes, Ph.B., and E. P. Griswold. Paper covers . 75

PHOTO NEWS YEAR BOOK OF PHOTOGRAPHY FOR 1890 . 50

WALDACK’S PHOTO ALMANAC . 25

THE LIGHTING IN PHOTOGRAPHIC STUDIOS.-By P. C. Duchochois. . . . 75

BROMIDE PAPER AND HOW TO USE IT. A practical treatise, written by

an expert, with a full-page illustration. Price, postpaid . 25 v

THE PHOTOGRAPHIC TIMES AHHDALS

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A Record of Photographic Progress.

I»rice, per copy, _______

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Contains five full-page illustrations —

An Exquisite Photo-Gravure, by Ernest Edwards.

A Bromide Rrint, by the Eastman Company.

A Silver Rrint, by Gustav Cramer, of St. Louis.

Two Mosstypes, by the Moss Engraving Company. 197 pages of Contributed. Matter consisting of articles on various subjects, by 80 repre¬ sentative photographic writers of this country and Europe.

Contains eight (8) full-page high-grade illustrations ; and over ninety (90) original con¬ tributions, written expressly for its pages, by the most eminent photographic writers of Europe and America.

THE ILLUSTRATIONS COMPRISE:

A Bhoto-Eithograph. showing an improved new process, by the Photo- Gravure Company of New York.

A Elioto-Copper-Elate Engraving of a Pictorial Landscape Subject, by E. Obernetter, of Munich.

A Meisenbach of “The Old Stone Bridge,” by Kurtz.

A X, inc Etching^, from the Engraving, which is itself as fine as an Engraving, by Stevens & Morris.

A Charming Cliild Rortrait, by Crosscup & West’s improved process. Three Mosstypes of popular subjects. And

330 PAGES OF VALUABLE INFORMATION.

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I Contains the Following Full-Page Pictorial Plates:

I <« Thomas Edison.*’ A Portrait of the Eminent Electrician. George M. Allen & Co., New York.

“ Babyhood.” A Tinted Photo-Gravure. The Photo-Gravure Co. of New York. “ Putnam’s Escape.” A Collection of Historic Views. The Crosscup & West Engraving Company, Philadelphia.

‘‘Southern Bruit.” An Orthochromatic Study. The Electro-Light Engraving Company, New York.

“ At the Barracks.” A copy of the great Meissonier picture. William Kurtz, N.Y. “ Minstrel Earty at ‘John Brown’s Fort.’ ” Photo-Engraving Com¬ pany, New York.

‘‘John Brown’s Home and Grave.” Lewis Engraving Co., Boston.

“ Off Duty.” An Instantaneous Study. William Kurtz, New York.

i ‘‘ Central Bark.” In the Menagerie. I,M Van Ness, New York.

‘‘A Merry Tale.” A Child Group. F. Gutekunst, Philadelphia.

“ The Van Rensselaer Manor House.” Photo-Electro Engraving Company, New York.

‘‘An Improvised Studio.” Electro-Tint Engraving Company, Philadelphia. “ The Bats.” A “Flash ” Light Photograph in Howe’s Cave. William Kurtz, N.Y. ‘‘A Raider’s Resort.” Morgan’s Favorite Rendezvous. M.Wolf, Dayton, Ohio. “ Group of Esquimaux.” William Kurtz, New York.

‘‘Diatoms.” Photo-Micrographs. William Kurtz, New York.

“ Tropical Luxuriance.” A Scene in Florida. Moss Engraving Co., N. Y. “ An Arctic Camp.” Moss Engraving Company, New York.

“ Home of Edgar Allen Foe.” Moss Engraving Company, New York.

\ NEARLY 400 PAGES OF READING MATTER.

vi

THE

American Annual of Photography

and Photographic Times Almanac

LARGER AND BETTER THAN EVER BEFORE.

Oyer TMty-six FULL-PAGE Illustrations. Oyer One Hundred Original Contributions.

PRICK THE SAME AS KSIJAK.

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SOME OF THE PICTORIAL ILLUSTRATIONS :

A Fine Copper-Plate Engraving (Portrait Study). By the New York Photo-Gravure Company.

“Attraction,” “Temptation,” “Satisfaction,” a series of three hunting pictures. By R. Eickemeyer, Jr.

The Solar Eclipse (December 22, 1889). By Prof. S. W. Burnham.

“Three Little Kittens.” By William M. Browne.

“ The County Fair.” By J. P. Davis.

A Portrait of Prof. Burnham. By Hill & Watkins.

“ I Love ’oo,” (a charming child picture). By Franklin Harper.

Daguerre Portraits. (Nine portraits of J. L. M. Daguerre, including one never before published.)

The Yacht “ Volunteer,” Before the Wind. By H. G. Peabody.

Finish of Race Between Taragon and St. Luke. By J. C. Hemment.

“ Enoch Arden.” A Portrait Study. By H. McMichael.

“ The Life Class.” By Charles N. Parker.

Portrait Study. By William Kurtz.

“ The Regatta.” Two Yachting Pictures. By A. Peebles Smith.

A “ Flash ” Picture. (Interior.) By Horace P. Chandler.

Old Mill on the Bronx River. By John Gardiner.

“ Sailing the High Seas Over.” By Harry Platt.

The Great Selkirk Glacier Face. By Alexander Henderson.

“Lightning.” (Two Pictures.) By W. N. Jennings.

“ Down in the Meadows.”

“ Forest Shadows.” By G. De Witt.

“In Chautauqua Woods.” By “A Chautauquan.”

Haines Falls. By W. S. Waterbury.

Besides many Pictures throughout the Advertising pages.

That Americans like the best of everything, and when the best costs the least they will buy it without urging.

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It goes without saying that a full-jeweled watch is worthy of a good case, and that an Encyclopedia should be bound in something more durable than paper covers.

The American Annual of Photography is now in world-wide favor, and commonly spoken of as an "Encyclopedia of Photographic Progress.”

It should be ordered with cloth binding (Library Edition), as it has, both in bulk and importance, outgrown paper covers. Other books, containing no more pages or information, sell for $3.00. In attractiveness they will not compare with

The Photographic Times Annual for 1891, which is the most profusely and handsomely illustrated Photographic Book ever published.

vii

IS

IT

NOT

SO?

“It makes this already valuable book simply invaluable.”

A CYCLOPEDIC INDEX

TO THE

AMERICAN ANNUAL OF PHOTOGRAPHY AND PHOTOGRAPHIC TIMES ALMANAC

FOR 189I.

CYCLOPEDIC INDEX.

Litmus — A coloring- matter derived from orcella tinctoria, a lichen. Its blue color turns red when in contact with acids. Alkalines restore the blue color again . . 142.

Lubricator . 348.

M

Magnesium — A metal of silvery white color, burns at a comparatively low temperature with extremely actinic and brilliant light . 207.

Magnesium Flash Light — Pure metallic magnesium reduced to fine powder. When blown forcibly through an intensely hot flame is instantaneously consumed and produces a highly actinic light. Originally the magnesium powder was mixed with substances evolv¬ ing oxygen, and when ignited produced a similar light _ 249, 207, 135.

Manganese — A metal of dusky white or grayish color, very hard and difficult to fuse . 176.

Mastic — The resinous exudation of Pistacia lentiscus , growing on the islands of the Grecian Archipelago, North Africa and Arabia. Yel¬ lowish white drops or tears, soluble in alcohol, chloroform, ether and benzole . 105.

Mayall. S. E.— A native of Ohio, dyer by trade. Embraced daguerreotyping at an early date. Established a studio in Philadelphia, but emigrated to England, where he and his descendants are carrying on a lucrative business to the present day . 28.

Meade. — We find the brothers Harry and Charles R. Meade in possession of a Daguerrean studio in Albany, N. Y., as early as 1842. They repaired to New York city a few years later and had the first elegantly and sumptuously fitted up gallery at 232 Broadway, where their pro¬ ductions stood foremost . 28.

Mercury Chloride, Mercuric Chloride, or Corrosive Sublimate. — A colorless, crystalline, semi-transparent mass, of metallic taste. It is soluble in water, alcohol and ether, and very poisonous . . 143.

Microscope — An optical instrument, consisting of a lens or combination of lenses for examining objects which are too minute to be viewed by the naked eye . 278, 205, 117, 107, 71.

Molecular — Belonging to, consisting of or residing in molecules . 59.

Monckhoven, Dr. D. von — Of Ghent, Belgium. Born 1834, died 1882 ; was an excellent chemist and physicist, who devoted himself chiefly to the scientific side of photography . 94

Monochrome— Of one color . 265.

Money Orders, International . 408.

Morse, Prof. Samuel F. B. — American inventor of the telegraph. One of the earliest experimenters in photography, and more successful than others of his contemporaries. H£ remained an ardent admirer and promoter of the art during the whole of his useful life . 28.

Mount Without Cockles . 348.

24 PAGES IK AI.I..

PRICE, . TEN CENTS.

For sale by all dealers in Photographic Materials, or sent post-paid on receipt of price by the publishers,

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The Modern Practice of Retouching Negatives . 75

The Photographic Times Annual for 1890 . 1 00

$6 75

It will thus be seen that if the books were bought separately they would cost $6.75, but purchased in the “Library” they cost only $5.25, and are neatly packed in a strong box. An appropriate and practical holiday gift for a photographer.

ix

TWELVE

FHOTOGRAFHIC

STUDIES.

A Collection of Photo-Gravures from Rep¬ resentative Negatives by Leading Photographic Artists in this Country and Abroad.

THE COLLECTION INCLUDES :

“ Dawn and Sunset”. . . .From the Negative by H. P. Robinson.

“Childhood” . “ “ H. McMichael.

“ As Age Steals On ”.. . . “ “ J. F. Ryder.

“ A Portrait Study ” . “ “ B. J. Falk.

“ Solid Comfort ” . “ “ John E. Dumont.

“ No Barrier ” . “ F. A. Jackson.

“ Still Waters ” . “ “ J. J. Montgomery.

“ Hi, Mister, May We Have Some Apples ! ”

From the Negative by Geo. B. Wood.

Printed on Japanese paper, mounted on boards. Size, 11x14, tied with silk cord in a specially designed cover and put up in a neat paper box.

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WILSON’S PHOTOGRAPHICS. — “ Chautauqua Edition,” with Appendix. By Edward L. Wilson, Ph.D. A most complete photographic lesson-book. Covers every department. 352 pages. Finely illustrated . 4 00

THE PROGRESS OF PHOTOGRAPHY.— By Dr. H. W. Vogel. Revised by Edward L. Wilson, Ph.D. Gives special consideration to Emulsion Photog¬ raphy, and has an additional chapter on Photography for Amateurs. Em¬ bellished with a full-page electric-light portrait by Kurtz, and seventy-two wood-cuts . 3 00

BIGELOW’S ARTISTIC PHOTOGRAPHY, with photographs . 4 00

HEARN’S STUDIES IN ARTISTIC PRINTING, with photographs . 3 00

BURNET’S HINTS ON ART. A facsimile reproduction of the costly original

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By W. T, Wilkinson. Revised and enlarged by Edward L. Wilson, Ph.D. Illustrated. 180 pages. Cloth bound . 3 00

WILSON’S PHOTOGRAPHIC MAGAZINE. A semi-monthly magazine, illus¬ trated by photographs, $5.00 a year ; club rate with Weekly Photographic Times, 9 00

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PHOTOGRAPHIC MOSAICS, 1891. Published annually. Cloth bound, $1.00;

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The illustrations are carefully selected, and represent the best work of repre¬ sentative American artists. There are also copies of famous pictures, from time to time, to illustrate lessons in art for photographers, accompanied by instructive reading matter.

The Editorials and Editorial Notes are of greatest practical value,

as they are the result of actual practice and experiment, by the staff.

Leading Articles by such acknowledged authorties as

Prof. W. K. BURTON, on Scientific and Prac¬ tical Subjects.

ANDREW PRINGLE, on Subjects of Greatest Value.

P. C. DUCHOCHOIS, on Chemical Subjects.

W. J. STILLMAN, on Art and Practical Sub¬ jects.

Prof. CHARLES EHRMANN, on Dark Room and Printing Practices.

And frequent contributions from G. Watmough Webster, F.C.S., of England; CarlSrna, Charles Scolik, Dr. Eder, Prof. Spitaler, Lieut.-Colonel Volkmer, of Austria; Dr. Lohse, Dr. Schnauss, KarlSchwier, Victor Schuman, and F. Mueller, of Germany.

Notes from the Every-day Gallery Experiences of such well-known Practical Photographers as W. H. Sherman, H. McMichael,

Prof. Karl Klauser, J. R. Swain,

J. M. Mora, and John Carbutt.

One Year,

Various Occasional Articles of a Practical Nature, and otherwise, by our favorite contributors:

Rev. C. E. Woodman, Ph.D., C. D. Cheney, D.D.S., Henry M. Parkhurst, Charles Wager Hull, C. W. Canfield, Rev. G. M. Searle, C. M. Brockway, Miss Adelaide Skeel.

“Occasional Notes,” by Prof. W. H. Pickering, of Harvard College Observatory.

The Chemistry of Photography, by W. Jerome Harrison, F.G.S.

Correspondence — Scientific and practical discussion of important and interesting questions, by practi¬ cal photographers, and letters from all parts of the globe, by intelligent and observing correspondents.

Notes and News,

Photographic Societies,

The Chautauqua School of Photography, Queries and Answers,

Our Editorial Table,

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xii

• PHOTOGRAPHIC

Printing Methods :

A PRACTICAL GUIDE

TO THE

Professional ^Amateur Worker,

BY THE

THIRD EDITION.

NEW YORK :

THE SCOVILL & ADAMS COMPANY, 423 Broome Street.

Copyright, 1887,

BY SCOVILL MANUFACTURING COMPANY.

Preface to the Third Edition

The appearance of a third edition of “ Printing Methods” affords another opportunity of thanking the fraternity for the very cordial reception accorded to this effort to make better known our gentle craft of photography.

The author has the best of reasons for believing that this little book has proved useful and helpful to many, and that it has therefore given a sufficient excuse for its existence.

Ho material changes have been made in this new edition. Beyond the correction of the errors of the first editions nothing has been altered.

The portrait and the brief biographical sketch of the second edition have been retained ; and the bromide print, by the Eastman Company, from an American film negative by Mr. W. J. Hickmott, of Hartford, Conn., and entitled “ Brook on Conway Meadows,” which appeared in the second edition, also reappears in this edition of the book.

PREFACE.

In the following pages the author has aimed to collect in easily accessible form, information and formulas connected with the production of photographic prints. His purpose has been to impart the information in the simplest and most prac¬ tical way possible, and to avoid errors in the numerous form¬ ulas given, all of which he believes will stand the test of ac¬ tual use. Sufficient material was collected to have filled double the number of pages of this little volume, but since to have done so would have been unduly to have increased the cost of the book without enhancing its usefulness, the writer has ex¬ ercised his best judgment in the selection of the material at his command, giving only those methods which his own prac¬ tice or that of others commended as useful and practical.

The work is rather one of compilation than of original re¬ search, and the author has not scrupled to make use of the work of others, giving due credit wherever the sources of in¬ formation were known to him.

The opening chapters on the “ Theory of Light” and its “Action on Sensitive Compounds” are merely condensed from “Abney’s Handbook of Photography,” and are given for the information of those who may care to know something of the chemical changes produced by the action of light upon the compounds most commonly used in photographic printing.

The author’s best thanks are due to Mr. W. I. Lincoln Adams and Dr. Charles Ehrmann, of the Photographic Times , for the very valuable advice and assistance which they have freely given him, and for their careful reading of the proof ; also to Mr. C. W. Canfield, for books furnished by him which were of great assistance in writing the chapters on “ Carbon Prints” and “ Photo-ceramics.”

In conclusion, the author ventures to express the hope that the following pages may prove useful to his brother amateurs to whom the book is respectfully dedicated.

W. H. Burbank.

Hewburgh, K Y., July, 1887.

List of Photographic Works Consulted.

Abney, W. de"W". — “ A Treatise on Photography.” New York, 1878. D. Appleton & Co.

Abney, W. de W., and Robinson, H. P. — “ The Art and Prac¬ tice of Silver Printing.” New York, 1881. Scovill Manufacturing Company.

Geymet. — “Traite Pratique des Emaux-Photographiques.” Paris, 1885. Gauthier- Yillars.

Geymet — “ Traite Pratique de Ceramique Photographique.” Paris, 1885. Gauthier- Yillars.

Geymet. — “ Traite Pratique de Photogravure sur Zinc et sur Cuivre.” Paris, 1886. Gauthier- Yillars.

Hardwich, T. F. — “ A Manual of Photographic Chemistry.”

New York, 1886. Scovill Manufacturing Co.

Husnik, J. — “ Die Heliographie.” Yienna, 1878. A. Hartleben.

Just, E. A. — “Der Positiv Process auf Gelatine-Emulsionpapier.” Yienna, 1885. E. A. Just.

Liesegang, Paul E. — “Le Procedeau Charbon.” Paris, 1886. Gauthier- Y illars.

Robinson, H. P. — “ Pictorial Effect in Photography.” Phila¬ delphia, 1881. Edward L. Wilson.

Roux, Y. — “ “ Traite Pratique de Photographie Decorative.” Paris, 1887. Gauthier-Yillars.

Roux, Y. — “Traite Pratique de Gravure Heliographique.” Paris, 1886. Gauthier-Yillars.

Roux, Y. — “ Manuel de Photographie et de Calcographie.” Paris, 1886. Gauthier-Yillars.

PHOTOGRAPHIC WORKS CONSULTED.

Vogel, H. W. — “ Progress in Photography.” Philadelphia, 1883. E. L. Wilson.

Wilson, E. L. — “ Photographies.” Philadelphia, 1883. E. L. Wilson.

Much valuable information has also been derived from vol¬ umes of the British Photographic Annuals, from Wilson’s “ Mosaics,” from the “ American Annual of Photography for 1887,” and from the columns of the Photographic Times and the other American Photographic journals, to all of which the author gratefully acknowledges his indebtedness.

Introduction

THEORY OF LIGHT.

The almost universally accepted theory of light, and the one which explains the greatest number of observed phenomena, is that which is known as the wave theory. Light as such is merely a sensation. All space is assumed to be permeated with a fluid known as ether, capable of being acted upon by a light or heat source in such a way as to give rise to an unbroken and continuous series of waves. Of the original form of these waves we know nothing. In the case of unpolarized, or ordinary white light, they are supposed to be, and the supposition seems to be substantiated by experiments, compounded of an infinite number of different undulations, each series differing in length from crest to crest. According as the length of these undula¬ tions vary, so do their effects differ. Those of a certain length, for instance, are able to affect the waves of the retina ; others affect nerves lying in the body, giving rise to the sensation of heat ; while others still are known only by their power of pro¬ ducing chemical combinations or decomposition in certain com¬ pounds.

The perception of color is due to the varying lengths of the light waves, the shortest gives the sensation of a violet color, the longest that of a brilliant red, waves of intermediate lengths producing respectively the sensation of blue, green, yellow, or orange. The limits of the heat spectrum are at least as great as those of the color spectrum, while the limits of the chemi¬ cally active rays are known to be much greater. The term actinic has been applied to all those rays capable of effecting decomposition in any compound, and their range varies for every ‘photographic substance, thus producing greater or less sensitiveness. It may be laid down as a fundamental and un¬ alterable law, that whenever light of any kind is absorbed by any body, work of some kind has been performed in that body. In the case of the compounds employed in photographic print-

INTRODUCTION.

ing, that work is some chemical or physical change or decom¬ position.

Action of Light Upon Sensitive Compounds.

In order to understand something of the changes produced in various sensitive compounds, some knowledge of the ultimate structure of matter is necessary. We may consider every particle of matter to be made up of molecules, each molecule being further subdivived into atoms, the smallest divisible por¬ tions of matter. The arrangement of these atoms differs in various substances. When, for instance, the atoms of any compound are so arranged as to be incapable of forming mole¬ cules of a simpler type, a large amount of work would be re¬ quired to separate them, and the atoms are said to occupy a position of stable equilibrium, such, for instance, as that of a pyramid standing on its base. Substances in which the atoms are in this state of exceedingly stable equilibrium, are, of course, useless for photographic printing purposes, and are said to be insensitive to light.

When, however, the atoms of a molecule are so arranged as to be capable of separating into more than one molecule, of less complex character it may be, it may happen that the atoms are in a condition of indifferent equilibrium, such, for instance, as that of the frustrum of a pyramid standing on a narrow section parallel to its base. Compounds in which the atoms are* in this state of indifferent equilibrium are, as a rule, easily affected by light, separating under its action, and arranging themselves in a different order. The sensitiveness of the molecules of such compounds to light depends upon the fact that the molecules are in a state of constant vibration. The effect of the successive impact of the waves of light is to increase the force of these vibrations until sufficient force is generated to cause the atoms to overcome the attraction bind¬ ing them together, when they arrange themselves in other groups, forming different compounds.

The final effect of the waves of light in breaking down the original arrangement of atoms, may be compared to the break¬ ing down of a bridge under the increasing vibrations imparted to it by a body of troops marching over it in regular step,

INTRODUCTION.

although the bridge might he capable of bearing double the weight. In both cases it is the regularity of the force com¬ municating the vibrations which produces the result. That this theory is correct is shown by the observed fact that the bodies employed for photographic purposed are chiefly affected by the shorter waves of light, the quickly repeated blows increasing more rapidly the force and extent of the vibrations, and pro¬ ducing a more rapid breaking up of the atoms.

One more remark may serve to explain why in some print¬ ing processes the change in atomic composition is so great as to be visible to the eye, as in the case of prints on the ordin¬ ary silver paper ; while in others, the change is so slight that the eye detects no alteration in physical appearance, as, for instance, in prints on bromide paper. This difference is due to the fact that the number of molecules affected in a brief interval of time is so small that the atomic change is invisible to the eye, or so like in physical appearance to the former condition as to escape detection until the application of the developer has rendered the change visible. The difference between the two images is not one of chemical composition, but merely of the number of molecules changed.

In the case of prints in silver the commonly accepted theory of the change produced by the action of light is that the molecule of silver chloride, Ag2Cl2, breaks up into one molecule of silver sub-chloride, Ag2Cl, and one of chlorine, Cl. But if silver chloride is exposed in the presence of free silver nitrate, as is the case with sensitized paper, then fresh silver chloride is formed and hypochlorous acid is liberated, which is a compound of oxygen and chlorine. In practice it has been found that the darkening of the silver chloride takes place much more rapidly when some chlorine absorbing sub¬ stance is present. Hence the common practice of fuming with ammonia, a chlorine absorber, although as vigorous prints may be produced by the addition of ammonium nitrate or potassium nitrate to the sensitizing bath.

It is to be remarked that the above theory of the action of light upon a sensitive silver surface, which is that of Captain Abney, of England, is disputed by many photographic chem¬ ists in this country. Professor Newberry, of Cornell Uni-

INTRODUCTION.

Yersity, a high authority, denies the existence of the sub¬ chloride, and claims that all the nitrate is converted into chloride. The subject is involved in great obscurity, and at present neither theory is to be implicitly accepted as the true one.

Many organic substances are capable of forming definite compounds with soluble silver salts, and the effect of the action of light is then made more complex. In the case of albumen paper sensitized on a solution of silver nitrate, an albuminate of silver is formed, and by the action of light the silver is reduced ro a condition of organic oxide, unaffected .by sodium hyposulphite, which dissolves the undarkened albuminate.

This is a brief statement of the action of light upon the compound of silver in common use by photographic printers, and it will serve to explain the changes produced in com¬ pounds of other metals occasionally employed for printing purposes, the ferric and uranic compounds being reduced to ferrous and uranous, which are amenable to the action of vari¬ ous developing agents. Salts of other metals are, as a rule, too insensitive to the action of light to be of value even for con¬ tact printing.

Resume of Printing Processes.

The fundamental principle underlying all the various meth¬ ods of photographic printing is that of molecular change pro¬ duced in the sensitive compound by the action of light. In most of the processes this change is visible in all its stages, subsequent manipulations only serving to change the color of the image and to give it greater permanence. To this class belong all the well-known printing-out methods, the general characteristic of all being the greater or less degree of visibil¬ ity of the impression when taken from the printing-frame ; the main point of difference being the nature of the sensitive medium employed, usually, iron, uranium, silver, or platinum and iron together.