A Combination, requiring the Use of only One Coating-box.--It is often wondered by beginners, why some solution requiring only one coating cannot be employed. This can be done, but the results are not so satisfactory as when two or more are employed. Such an accelerator may be produced by adding alcoholic solution of iodine to a solution of chlorate of potash, until the latter will take up no more of the former, and to each ounce, by measure of this solution, ten drops of a saturated solution of bromide in water are added. The solution of chlorate of potash is made by diluting, one part of a saturated solution of the salt with ten parts of water. The use of the chlorate is simply as a solvent of iodine.

Fats as Accelerators.--The use of fats, oils, or greasy substances, has been one of the most emphatic prohibitions about the Daguerreotype plate. Yet it has been proved that its presence in a small quant.i.ty upon the silver surface has the effect of reducing the time of exposure in the camera from two-thirds to three-fourths. An application may be made as follows: Pour sweet oil, or rub beef or mutton fat, on a common buff, which is free from all polishing powders. With this, buff a well-cleaned plate, and it will leave a sc.u.m, which should be mostly removed by using another buff, which should be clean. Coat the plate in the usual manner, and the result will be a great reduction in the time of exposure in the camera. The impression produced upon a plate so prepared presents, when coming from the vapor of mercury, a grey, sc.u.mmy appearance, which, on the application of heat in gilding, does not improve; hence its use is not generally adopted.

We have inst.i.tuted some investigations upon this subject, and in the present volume, we shall not refer to it further. Those wishing to learn more fully the effect of light upon organic substances will find Robert Hunt's "Researches on Light" an invaluable work.

LIGHT AND OPTICS.

CHAPTER IV.

Light--Optics--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.

It is advisable that persons engaging in the Daguerreotype art should have at least a little knowledge of the general principles of light and optics. It is not the author's design here to give a full treatise on these subjects, but he only briefly refers to the matter, giving a few facts.

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 s.p.a.ce, and the various modifications it undergoes at the surfaces and interior of terrestrial substances.

Light is white and colorless, as long as it does not come in contact with matter. When in apposition with any body, 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.

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, it will form a white round spot upon the place where it falls. If a triangular prism of gla.s.s 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.

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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.

[Ill.u.s.tration: Fig. 1 (amdg_1.gif)]

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 pa.s.sing 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 {134} white light besides the prism, of which one of the princ.i.p.al and most interesting to the Daguerreotypist 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 prevents the beautifully-colored image we see upon the ground gla.s.s in our cameras.

[Ill.u.s.tration: Fig. 2 (amdg_2.gif)]

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 ill.u.s.tration, Fig. 2, will readily bring to the mind 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, to which gradually becomes colorless; similarly, red light exists beyond the a.s.signed 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.

(Blue paper and blue color have been somewhat extensively used by our Daguerreotype 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 gla.s.s, 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 burning charcoal, which is the princ.i.p.al light from candles, oil and gas, contains three rays--red, yellow, and blue. The dazzling light emitted from lime intensely heated, known as the Drummond light, gives the colors of the prism almost as bright as the solar spectrum.

If we expose a prepared Daguerreotype plate or sensitive paper to the solar spectrum, it will be observed that the luminous power (the yellow) occupies but a small s.p.a.ce compared with the influence of heat and chemical power. R. Hunt, in his Researches on Light, has presented the following remarks upon the accompanying ill.u.s.tration:

[Ill.u.s.tration: Fig. 3 (amdg_3.gif)]

"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.

A ray of light, in pa.s.sing obliquely through any medium of uniform density, does not change its course; but if it should pa.s.s 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.

[Ill.u.s.tration: Fig. 4 (amdg_4.gif)]

It is observed that it proceeds in a less oblique direction (towards the dotted line), and, on pa.s.sing on through, leaves the liquid, proceeding in a line parallel to that at 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 pa.s.ses 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.

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

[Ill.u.s.tration: Fig. 5, 6, 7 (amdg_5.gif)]

Fig 5 represents 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.

If parallel rays of light fall upon a double-convex lens, D D, Fig. 8, they will be refracted (excepting such as pa.s.s directly through the centre) to a point termed the princ.i.p.al focus.

[Ill.u.s.tration: Fig. 8a (amdg_8a.gif)]

The lines A B C represent parallel rays which pa.s.s through the lens, D D, and meet at F; this point being the princ.i.p.al 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 gla.s.s 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 should 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."

[Ill.u.s.tration: Fig. 9 (amdg_9.gif)]

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 ill.u.s.trate this more clearly, I will refer to the following woodcut:

[Ill.u.s.tration: Fig. 10 (amdg_10.gif)]

"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 proceding 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 all 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.

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 gla.s.s, 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 const.i.tutes 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."

[Ill.u.s.tration: Fig. 11 (amdg_11.gif)]