Theory of Light
THE THEORY ON LIGHT.
THE PHOTOGRAPHIC PRINCIPLE
Some philosophers contend that to the existence of light alone we owe the beautiful effects produced by the Photogenic art, while others give sufficient reasons for doubting the correctness of the assumption. That the results are effected by a principle associated with light and not by the luminous principle itself, is the most probable conclusion. The importance of a knowledge of this fact becomes most essential in practice, as will presently be seen. To this principle Mr. Hunt gives the name of ENERGIA.
THE NATURE of Light is not wholly known, but it is generally believed to be matter, as in its motions it obeys the laws regulating matter. So closely is it connected with heat and electricity that there can be little doubt of their all being but different modifications of the same substance. I will not, however, enter into a statement of the various theories of Philosophers on this head, but content myself with that of Sir Isaac Newton; who supposed rays of light to consist of minute particles of matter, which are constantly emanating from luminous bodies and cause vision, as odoriferous particles, proceeding from certain bodies, cause smelling.
The effects of light upon other bodies, and how light is effected by them, involve some of the most important principles, which if properly understood by Daguerreotypists would enable them to improve and correct many of the practical operations in their art. These effects we shall exhibit in this and the following chapters. Before we enter on this subject it will be necessary to become familiar with the
DEFINITIONS of some of the terms used in the science of optics.
Luminous bodies are of two kinds; those which shine by their own light, and those which shine by reflected light.
Transparent bodies are such as permit rays of light to pass through them.
Translucent bodies permit light to pass faintly, but without representing the figure of objects seen through them.
Opaque bodies permit no light to pass through them, but reflect light.
A ray is a line of light.
A beam is a collection of parallel rays.
A pencil is a collection of converging, or diverging rays.
A medium is any space through which light passes.
Incident rays are those which fall upon the surface of a body.
Reflected rays are those which are thrown off from a body.
Parallel rays are such as proceed equally distant from each other through their whole course.
Converging rays are such as approach and tend to unite at any one point, as at b. Fig. 3.
Diverging rays are those which continue to recede from each other, as at e. Fig. 3.
A Focus is that point at which converging rays meet.
MOTION OF LIGHT--Rays of light are thrown off from luminous bodies in every direction, but always in straight lines, which cross each other at every point; but the particles of which each ray consists are so minute that the rays do not appear to be impeded by each other. A ray of light passing through an aperture into a dark room, proceeds in a straight line; a fact of which any one may be convinced by going into a darkened room and admitting light only through a small aperture.
Fig. 1 (HIPHO_1.GIF)
Light also moves with great velocity, but becomes fainter as it recedes from the source from which it eminates; in other words, diverging rays of light diminish in intensity as the square of the distance increases. For instance let a fig. 1, represent the luminous body from which light proceeds, and suppose three square boards, b. c. d. severally one, four and sixteen square inches in size be placed; b one foot, c two feet, and d four feet from a, it will be perceived that the smallest board b will throw c into shadow; that is, obstruct all rays of light that would otherwise fall on c, and if b were removed c would in like manner hide the light from d--Now, if b recieve as much light as would fall on c whose surface is four times as large, the light must be four times as powerful and sixteen times as powerful as that which would fall on the second and third boards, because the same quantity of light is diffused over a space four and sixteen times greater. These same rays may be collected and their intensity again increased.
Rays of light are reflected from one surface to another; Refracted, or bent, as they pass from the surface of one transparent medium to another; and Inflected, or turned from their course, by the attraction of opaque bodies. From the first we derive the principles on which mirrors are constructed; to the second we are indebted for the power of the lenses, and the blessings of sight,--for the light acts upon the retina of the eye in the same manner as on the lens of a camera. The latter has no important bearing upon our subject.
When a ray of light falls perpendicularly upon an opaque body, it is reflected bark in the same line in which it proceeds; in this case the reflected ray returns in the same path the incident ray traversed; but when a ray falls obliquely, it is reflected obliquely, that is, it is thrown off in opposite direction, and as far from the perpendicular as was the incident ray, as shown at Fig. 2; a representing the incident ray and b the reflected. The point, or angle c made by the incident ray, at the surface of the reflector e f, with a line c d, perpendicular to that surface, is called the angle of incidence, while the angle formed by the reflected ray b and the perpendicular line d is called the angle of reflection, and these angles are always equal.
Fig. 2 (HIPHO_2.GIF)
It is by this reflection of light that objects are made visible; but unless light falls directly upon the eye they are invisible, and are not sensibly felt until after a certain series of operations upon the various coverings and humors of the eye. Smooth and polished surfaces reflect light most powerfully, and send to the eye the images of the objects from which the light proceeded before reflection. Glass, which is transparent--transmitting light--would be of no use to us as a mirror, were it not first coated on one side with a metalic amalgam, which interrupts the rays in their passage from the glass into the air, and throws them either directly in the incident line, or in an oblique direction. The reason why trees, rocks and animals are not all mirrors, reflecting other forms instead of their own, is, that their surfaces are uneven, and rays of light reflected from an uneven surface are diffused in all directions.
Parallel rays falling obliquely upon a plane mirror are reflected parallel; converging rays, with the same degree of convergence; and diverging rays equally divergent.
Stand before a mirror and your image is formed therein, and appears to be as far behind the glass as you are before it, making the angle of reflection equal to that of incidence, as before stated. The incident ray and the reflected ray form, together, what is called the passage of reflection, and this will therefore make the actual distance of an image to appear as far again from the eye as it really is. Any object which reflects light is called a radiant. The point behind a reflecting surface, from which they appear to diverge, is called the virtual focus.
Rays of light being reflected at the same angle at which they fall upon a mirror, two persons can stand in such a position that each can see the image of the other without seeing his own. Again; you may see your whole figure in a mirror half your length, but if you stand before one a few inches shorter the whole cannot be reflected, as the incident ray which passes from your feet into the mirror in the former case, will in the latter fall under it. Images are always reversed in mirrors.
Convex mirrors reflect light from a rounded surface and disperse the rays in every direction, causing parallel rays to diverge, diverging rays to diverge more, and converging rays to converge less--they represent objects smaller than they really are--because the angle formed by the reflected ray is rendered more acute by a convex than by a plane surface, and it is the diminishing of the visual angle, by causing rays of light to be farther extended before they meet in a point, which produces the image of convex mirrors. The greater the convexity of a mirror, the more will the images of the objects be diminished, and the nearer will they appear to the surface. These mirrors furnish science with many curious and pleasing facts.
Concave mirrors are the reverse of convex; the latter being rounded outwards, the former hollowed inwards--they render rays of light more converging--collect rays instead of dispersing them, and magnify objects while the convex diminishes them.
Rays of light may be collected in the focus of a mirror to such intensity as to melt metals. The ordinary burning glass is an illustration of this fact; although the rays of light are refracted, or passed through the glass and concentrated into a focus beneath.
When incident rays are parallel, the reflected rays converge to a focus, but when the incident rays proceed from a focus, or are divergent, they are reflected parallel. It is only when an object is nearer to a concave mirror than its centre of concavity, that its image is magnified; for when the object is farther from the mirror, this centre will appear less than the object, and in an inverted position.
The centre of concavity in a concave mirror, is an imaginary point placed in the centre of a circle formed by continuing the boundary of the concavity of the mirror from any one point of the edge to another parallel to and beneath it.
REFRACTION OF LIGHT:--I now pass to the consideration of the passage of light through bodies.
A ray of light failing perpendicularly through the air upon a surface of glass or water passes on in a straight line through the body; but if it, in passing from one medium to another of different density, fall obliquely, it is bent from its direct course and recedes from it, either towards the right or left, and this bending is called refraction; (see Fig. 3, b.) If a ray of light passes from a rarer into a denser medium it is refracted towards a perpendicular in that medium; but if it passes from a denser into rarer it is bent further from a perpendicular in that medium. Owing to this bending of the rays of light the angles of refraction and incidence are never equal.
Transparent bodies differ in their power of bending light--as a general rule, the refractive power is proportioned to the density--but the chemical constitution of bodies as well as their density, is found to effect their refracting power. Inflammable bodies possess this power to a great degree.
The sines of the angle of incidence and refraction (that is, the perpendicular drawn from the extremity of an arc to the diameter of a circle,) are always in the same ratio; viz: from air into water, the sine of the angle of refraction is nearly as four to three, whatever be the position of the ray with respect to the refracting surface. From air into sulphur, the sine of the angle of refraction is as two to one--therefore the rays of light cannot be refracted whenever the sine of the angle of refraction becomes equal to the radius* of a circle, and light falling very obliquely upon a transparent medium ceases to be refracted; this is termed total reflection.
* The RADIUS of a circle is a straight line passing from the centre to the circumference.
Since the brightness of a reflected image depends upon the quantity of light, it is quite evident that those images which arise from total reflection are by far the most vivid, as in ordinary cases of reflection a portion of light is absorbed.
I should be pleased to enter more fully into this branch of the science of optics, but the bounds to which I am necessarily limited in a work of this kind will not admit of it. In the next chapter, however, I shall give a synopsis of Mr. Hunt's treatise on the "Influence of the Solar Rays on Compound Bodies, with especial reference to their Photographic application"--a work which should be in the hands of every Daguerreotypist, and which I hope soon to see republished in this country. I will conclude this chapter with a brief statement of the principles upon which the Photographic art is founded.
SOLAR and Stellar light contains three kinds of rays, viz:
1. Colorific, or rays of color.
2. Calorific, or rays of heat.
3. Chemical rays, or those which produce chemical effects.
On the first and third the Photographic principle depends. In explaining this principle the accompanying wood cuts, (figs. 3 and 4) will render it more intelligible.
If a pencil of the sun's rays fall upon a prism, it is bent in passing through the transparent medium; and some rays being more refracted than others, we procure an elongated image of the luminous beam, exhibiting three distinct colors, red, yellow and blue, which are to be regarded as primitives--and from their interblending, seven, as recorded by Newton, and shown in the accompanying wood cut. These rays being absorbed, or reflected differently by various bodies, give to nature the charm of color. Thus to the eve is given the pleasure we derive in looking upon the green fields and forests, the enumerable varieties of flowers, the glowing ruby, jasper, topaz, amethist, and emerald, the brilliant diamond, and all the rich and varied hues of nature, both animate and inanimate.
Fig. 3 (HIPHO_3.GIF)
Now, if we allow this prismatic spectrum (b. Fig. 3.) to fall upon any surface (as at c.) prepared with a sensitive photographic compound, we shall find that the chemical effect produced bears no relation to the intensity of the light of any particular colored ray, but that, on the contrary, it is dispersed over the largest portion of the spectrum, being most energetic in the least luminous rays, and ever active over an extensive space, where no traces of light can be detected. Fig. 4, will give the student a better idea of this principle. It is a copy of the kind of impression which the spectrum, spoken of, would make on a piece of paper covered with a very sensitive photographic preparation. The white space a. corresponds with the most luminous, or yellow ray, (5, Fig. 3) over limits of which all chemical change is prevented. A similar action is also produced by the lower end of the red ray c; but in the upper portion, however we find a decided change (as at d). The most active chemical change, you will perceive, is produced by the rays above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being the least active, and the blue (3) and violet (1) rays the most so, the action still continuing far beyond the point b which is the end of the luminous image.
Fig. 4 (HIPHO_4.GIF)
Suppose we wish to copy by the Daguerreotype, or Calotype process, any objects highly colored--blue, red and yellow, for instance predominating--the last of course reflects the most light, the blue the least; but the rays from the blue surface will make the most intense impression, whilst the red radiations are working very slowly, and the yellow remains entirely inactive. This accounts for the difficulty experienced in copying bright green foliage, or warmly colored portraits; a large portion of the yellow and red rays entering into the composition of both--and the imperfections of a Daguerreotype portrait of a person with a freckled face depends upon the same cause.
A yellow, hazy atmosphere, even when the light is very bright, will effectually prevent any good photographic result--and in the height of summer, with the most sensative process, it not unfrequently happens that the most annoying failures arise from this agency of a yellow medium. A building painted of a yellow color, which may reflect the sun's rays directly into the operator's room will have the same effect. Daguerreotypists, being ignorant of these facts, are very apt to charge their want of success to the plates, or chemicals, or any thing but the real cause; and it would be well to bear these facts constantly in mind and as far as possible avoid them. This, may be accomplished, in a measure, by a choice of location or by having the glass of your windows tinged with blue; or a screen of thin blue paper may be interposed between the light and sitter. In selecting subjects, all striking contrasts in color should be avoided, and sitters for portraits should be cautioned not to wear anything that may produce the effect spoken of--dark dresses always being the best.
The action of light both combines and decomposes bodies. For instance, chlorine and hydrogen will remain in a glass vessel without alteration if kept in the dark; but if exposed to the rays of the sun, they immediately enter into combination, and produce hydrochloric acid. On the other hand, if colorless nitric acid be exposed to the sun, it becomes yellow, then changes to red, and oxygen is liberated by the partial decomposition effected by the solar rays.
Of the organic substances none are more readily acted upon by light than the various combinations of silver.
Of these some are more, and others less sensitive. If Chloride of silver, which is a white precipitate formed by adding chloride of sodium (common salt) to a solution of nitrate of silver, be exposed to diffused light, it speedily assumes a violet tint, and ultimately becomes nearly black. With iodide of silver, bromide of silver, ammonio-nitrate of silver, and other salts of this metal, the result will be much the same.
Some bodies, which under the influence of light, undergo chemical changes, have the power of restoring themselves to their original condition in the dark. This is more remarkably displayed in the iodide of platinum, which readily recieves a photogenic image by darkening over the exposed surfaces, but speedily loses it by bleaching in the dark. The ioduret of Daguerre's plate, and some other iodides, exhibit the same peculiarity--This leads us to the striking fact, that bodies which have undergone a change of estate under the influence of day-light have some latent power by which they can renovate themselves. Possibly the hours of night are as necessary to inanimate nature as they are to the animate. During the day, an excitement which we do not heed, unless in a state of disease, is maintained by the influence of light and the hours of repose, during which the equilibrium is restored, are absolutely necessary to the continuance of health.
Instead of a few chemical compounds of gold and silver, which at first were alone supposed to be photographic, we are now aware that copper, platinum, lead, nickel, and indeed, probably all the elements, are equally liably to change under the sun's influence. This fact may be of benefit to engravers, for if steel can be made to take photographic impressions, the more laborious process of etching may be dispensed with. In fact, in the latter part of this work, a process is described for etching and taking printed impressions from Daguerreotype plates. As yet this process has produced no decided beneficial results--but future experiments may accomplish some practical discovery of intrinsic value to the art of engraving.
A very simple experiment will prove how essential light is to the coloring of the various species comprising the vegetable and animal kingdoms. If we transplant any shrub from the light of day into a dark cellar, we will soon see it lose its bright green color, and become perfectly white.
Another effect of light is that it appears to impart to bodies some power by which they more readily enter into chemical combination with others. We have already said that chlorine and hydrogen, if kept in the dark, will remain unaltered; but if the chlorine alone be previously exposed to the sun, the chlorine thus solarised will unite with the hydrogen in the dark. Sulphate of iron will throw down gold or silver from their solutions slowly in the dark; but if either solution be first exposed to sunshine, and the mixture be then made, in the dark, the precipitation takes place instantly. Here is again, evidence of either an absorption of some material agent from the sunbeam, or an alteration in the chemical constitution of the body. It was from understanding these principles and applying them that philosophers were enabled to produce the Calotype, Daguerreotype, &c. For the effects and action of light on the camera, see Chapter V.
Some advances have been made towards producing Photographic impressions in color--the impossibility of which some of our best and oldest artists have most pertinaciously maintained. The colored image of the spectrum has been most faithfully copied, ray for ray, on paper spread with the juice of the Cochorus Japonica, (a species of plant) and the fluoride of silver; and on silver plate covered with a thin film of chloride. The day may be still remote when this much to be desired desideratum shall be accomplished in portrait taking; but I am led to hope that future experiments may master the secret which now causes it to be looked upon, by many, as an impossibility.
That great advantages have resulted, and that greater still will result from the discovery of the Photographic art, few will deny. The faithful manner in which it copies nature, even to the most minute details, renders it of much value to the painter; but a few minutes sufficing to take a view that formerly would have occupied several days. Its superiority in portraits, over miniature or oil painting has been tacitly acknowledged by the thousands who employ it to secure their own, or a friends likeness, and by the steady increase in the number of artists who are weekly, aye daily springing up in every town and village in the land.