A Practical Manual of the Collodion Process, Giving in Detail a Method For Producing Positive and Negative Pictures on Glass and Paper. — Themes and Context

Introduction--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 Sunbeam--Refraction--Reflection--Lenses--Copying--Spherical Aberration--Chromatic Aberration 13

Camera--Arrangement of Lenses--Camera Tubes--Camera Boxes, Bellows, and Copying--Camera Stands--Head Rests--Cleaning Vice--Nitrate Bath--Leveling Stands--Printing Frames--Collodion Vials 26

=Practical Hints on Photographic Chemistry.=

Soluble Cotton--Manipulation--Plain Collodion--Bromo-Iodized Collodion for Positives--Ditto for Negatives--Solution of Bromide and Iodide of Potassium and Silver--Double Iodide of Potassium and Silver--Developing Solution--Fixing the Solution--Brightening and Finishing the Image--Photographic Chemicals 41

=Practical Details of the Positive or Ambrotype Process.=

Lewis's Patent Vices for Holding the Glass--Cleaning and Drying the Glass--Coating--Exposure in the Camera--Developing--Fixing or Brightening--Backing up, &c. 129

=Practical Details of the Negative Process.=

Negative Process--Soluble Cotton--Plain Collodion--Developing Solution--Re-Developing Solution--Fixing the Image--Finishing the Image--Nitrate of Silver Bath 143

=Practical Details of the Printing Process.=

Printing Process--Salting Paper--Silvering Paper--Printing the Positive--Fixing and Coloring Bath--Mounting the Positive--Facts worth Knowing 151

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

The Collodio-Albumen Process in Detail 190

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

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.

To those who are unacquainted with the theory of light (and for their benefit this chapter is given), it may be a matter of wonder how a beam of light can be divided. This can be understood when I say, that white light is a bundle of colored rays united together, and when so incorporated, they are colorless; but in passing _through_ the prism the bond of union is severed, and the colored rays come out _singly_ and _separately_, because each ray has a certain amount of refracting (bending) power, peculiar to itself. These rays always hold the same relation to each other, as may be seen by comparing every spectrum or rainbow; there is never any confusion or misplacement.

There are various other means of decomposing white light besides the prism, of which one of the principal and most interesting to the photographer, is by _reflection_ from colored bodies. If a beam of white light falls upon a white surface, it is reflected without change; but if it falls upon a _red_ surface, only the red ray is reflected: so also with yellow and other colors; the ray which is reflected corresponds with the color of the object. It is this reflected decomposed light which presents the beautifully colored image we see upon the ground glass in our cameras.

[Sidenote: LIGHT, HEAT, AND ACTINISM.]

A sunbeam may be capable of three divisions--light, heat, and actinism; the last causes all the chemical changes, and is the acting power upon surfaces prepared to receive the photographic image. The accompanying illustration, Fig. 2, will readily bring to the minds of the reader the relation of these one to another, and their intensities in the different parts of a decomposed sunbeam.

The various points of the solar spectrum are represented in the order in which they occur between A and B, this exhibits the limits of the Newtonian spectrum, corresponding with Fig. 1. Sir John Herschel and Seebeck have shown that there exists, beyond the violet, a faint violet light, or rather a _lavender_, to _b_, which gradually becomes colorless; similarly, red light exists beyond the assigned limits of the red ray to _a_. The greatest amount of actinic power is shown at E opposite the violet; hence this color "exerts" the greatest amount of influence in the formation of the photographic image.

[Sidenote: COLORING WALLS BLUE.]

(Blue paper and blue color have been somewhat extensively used by our operators in their operating rooms and skylights, in order to facilitate the operation in the camera. I fancy, however, that this plan cannot be productive of as much good as thought by some, from the fact, that the light falling upon the subject, and then reflected into the camera, is, coming through colorless glass, not affected by such rays as may be reflected from the walls of the operating room; and even if it were so, I conceive that it would be injurious, by destroying the harmony of Shadows which might otherwise occur.) The greatest amount of white light is at C; the yellow contains less of the chemical power than any Other portion of the solar Spectrum. It has been found that the most intense heat is at the _Extreme Red_, _b_.

Artificial lights differ in their color; the white light of turning charcoal, which is the principal light from candles, oil and gas, contains three fays--red, yellow and blue. The dazzling light emitted from lime intensely heated, known as the _Drummond light_, gives the color of the prism almost as bright as the solar spectrum.

[Sidenote: LIGHT, HEAT, AND CHEMICAL POWER.]

If we expose a prepared collodionized plate or sensitive paper to the solar spectrum, it will be observed that the luminous power (the yellow) occupies but a small space compared with the influence of heat and chemical power. R. Hunt, in his _Researches on Light_, has presented the following remarks upon the accompanying illustration:--

"If the linear measure, or the diameter of a circle which shall include the luminous rays, is 25, that of the calorific spectrum will be 42·10, and of the chemical spectrum 55·10. Such a series of circles may well be used to represent a beam from the sun, which may be regarded as an atom of _Light_ surrounded with an invisible atmosphere of _Heat_, and another still more extended, which possesses the remarkable property of producing chemical and molecular change."

[Sidenote: REFRACTION.]

A ray of light, in passing obliquely through any medium of uniform density, does not change its course; but if it should pass into a denser body, it would turn from a straight line, pursue a less oblique direction, and in a line nearer to a perpendicular to the surface of that body. Water exerts a stronger refracting power than air; and if a ray of light fall upon a body of this fluid its course is changed, as may be seen by reference to Fig. 4. It is observed that it proceeds in a less oblique direction (towards the dotted line), and, on passing on through, leaves the liquid, proceeding in a line parallel to that which it entered. It should be observed, that at the _surface_ of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body. The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface. It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture.

When a ray of light passes from one medium to another, and through that into the first again, if the two refractions be equal, and in opposite directions, no sensible effect will be produced.

[Sidenote: REFRACTION, LENSES, FOCUS.]

The reader may readily comprehend the phenomena of refraction, by means of light passing through lenses of different curves, by reference to the following diagrams:--

Fig. 5 representing a double-convex lens, Fig. 6 a double-concave, and Fig. 7 a concavo-convex or meniscus. By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.

[Sidenote: ENLARGING OR REDUCING IN COPYING.]

If parallel rays of light fall upon a double-convex lens, D D, Fig. 8, they will be refracted (excepting such as pass directly through the centre) to a point termed the principal focus. The lines A B C represent parallel rays which pass through the lens D D, and meet at F; this point being the principal focus, its distance from the lens is called the _focal length_. Those rays of light which are traversing a _parallel_ course, when they enter the lens are brought to a focus nearer the lens than others. Hence the difficulty the operator sometimes experiences by not being able to "obtain a focus," when he wishes to secure a picture of some very distant objects; he does not get his _ground glass near enough to the lenses_. Again, the rays from an object near by may be termed diverging rays. This will be better comprehended by reference to Fig. 9, where it will be seen that the dotted lines, representing parallel rays meet nearer the lenses than those from the point A. The closer the object is to the lenses, the greater will be the divergence. This rule is applicable to copying, Did we wish to copy a 1/6 size daguerreotype on a 1/16 size plate, we would place it in such a position to the lenses at A, that the focus would be at F, where the image would be represented at about the proper size. Now, if we should wish to copy the 1/6 size picture, and produce another of exactly the same dimensions, we have only to bring it nearer to the lenses, so that the lens D E shall be equi-distant from the picture and the focus, _i. e._ from A to B. The reason of this is, that the distance of the picture from the lens, in the last copy, is less than the other, and the divergence has increased, throwing the focus further from the lens.

These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying.

I would remark that the points F and A, in Fig. 9, are termed "conjugate foci."

If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it. To illustrate this more clearly, I will refer to the following wood-cut:--

"If A B C is an object placed before a convex lens, L L, every point of it will send forth rays in all directions; but, for the sake of simplicity, suppose only three points to give out rays, one at the top, one at the middle, and one at the bottom; the whole of the rays then that proceed from the point A, and fall on the lens L L, will be refracted and form an image somewhere on the line A G E, which is drawn direct through the centre of the lens; consequently the focus E, produced by the convergence of the rays proceeding from A, must form an image of A, only in a different relative position; the middle point of C, being in a direct line with the axis of the lens, will have its image formed on the axis F, and the rays proceeding from the point B will form an image at D; so that by imagining luminous objects to be made up of an infinite number of radiating points, and the rays from each individual point, although falling on the whole surface of the lens, to converge again and form a focus or representation of that point from which the rays first emerged, it will be very easy to comprehend how images are formed, and the cause of those images being reversed.

"It must also be evident, that in the two triangles A G B and D G E, that E D, the length of the image, must be to A B, the length of the object, as G D, the distance of the image, is to G B, the distance of the object from the lens."

[Sidenote: SPHERICAL ABERRATION.]

It will be observed, that in the last cut the image produced by the lens is curved. Now, it would be impossible to produce a well-defined image from the centre to the edge upon a _plain_ surface; the outer edges would be misty, indistinct, or crayon-like. The centre of the image might be represented clear and sharp on the ground glass, yet this would be far from the case in regard to the outer portions. This is called _spherical aberration_, and to it is due the want of distinctness which is frequently noticed around the edges of pictures taken in the camera. To secure a camera with a _flat, sharp field_, should be the object of every Operator; and, in a measure, this constitutes the great difference in cameras manufactured in this country.

Spherical aberration is overcome by proper care in the formation of the lens:--"It can be shown upon mathematical data that a lens similar to that given in the following diagram--one surface of which is a section of an ellipse, and the other of a circle struck from the furthest of the two foci of that ellipse--produces no aberration.

"At the earliest period of the employment of the camera obscura, a _double-convex_ lens was used to produce the image; but this form was soon abandoned, on account of the spherical aberration so caused. Lenses for the photographic camera are now always ground of a concavo-convex form, or meniscus, which corresponds more nearly to the accompanying diagram."

[Sidenote: CHROMATIC ABERRATION.]

_Chromatic Aberration_ is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (14) a beam of light is decomposed by passing through a glass prism giving seven distinct colors--_red_, _orange_, _yellow_, _green_, _blue_, _indigo_ and _violet_.

Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut.

If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays, each having a different _index_ of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to _r_, and the violet rays, being the most refrangible, to _v_: the distance _v r_ constitutes the chromatic aberration, and the circle, of which the diameter is _a l_, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and _o_, so as to cut the cone L _a l_ L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L _a l_ L, of which the external rays L _a_ L _l_, are red; if the screen be moved to the other side of _o_, the luminous circle will be bordered with violet, because it will be a section of the cone M _a_ M _l_, of which the exterior rays are violet. To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pass which are at the edge of the glass; a violet image of the sun will then be seen at _v_, red at _r_, and, finally, images of all the colors of the spectrum in the intermediate space; consequently, the general image will not only be confused, but clothed with prismatic colors.

To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from glass possessing different refrangible powers, so that the rays of light passing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.

To the photographer one of the most important features, requiring his particular attention, is, that he be provided with a good lens. By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is _occasionally_ subjected. We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the _spherical_ or _chromatic aberration_.

CAMERA--ARRANGEMENT OF LENSES--CAMERA TUBES--CAMERA-BOXES, BELLOWS, AND COPYING--CAMERA STANDS--HEAD RESTS--CLEANING VICE--NITRATE BATH--LEVELING STANDS--PRINTING FRAMES--COLLODION VIALS.

Babtista Porta, when he saw for the first time, on the walls of his dark chamber, the images of external nature, pictured by a sunbeam which found its way through only a small hole, little thought of the importance which would be attached to the instrument he was, from this cause, led to invent. The camera obscura of this Italian philosopher remained as a mere scientific toy for years, and it was not until Daguerre's discovery that its true value was estimated. It now plays a very important part in giving employment to at least _ten thousand persons_ in this country alone.

It is of the utmost importance, in selecting a set of apparatus, to secure a good camera; for without such no one can obtain fine pictures. In testing it, see that it gives the pupil of the eye and lineaments of the features sharp and distinct; and that the whole image on the ground glass has a fine pearly hue. Look also to the field, and observe that the focus is good at the centre and extreme edges of the ground glass, at the same time. A poor camera generally gives a misty image, with the lights and shades apparently running together. The best American cameras are fully equal to those imported, while they cost much less; but there are great numbers sold which are not worth using.

[Sidenote: CAMERA TUBES AND LENSES.]

If a lens gives a well defined image on the ground glass, it should do the same on the plate. Many a valuable lens has been condemned for failing in this, merely in consequence of the plate-holder not being in focus with the ground-glass. In case of deficiency in this, put a glass into the holder, lay a rule across the face, and measure the distance between them very exactly; measure the ground-glass in the same way, and make the distance agree perfectly, by moving the ground-glass either back or forward in the frame, as the case may be, so that the surface of the glass plate shall occupy precisely the same position as the face of the ground-glass when in the camera.

[Sidenote: POSITION OF LENSES IN THE TUBE.]