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whose refrangibility has more recently been ascertained by M. Rudberg. Thus, in the case of seven rays in each of twenty different substances, solid and fluid, the dispersion of light takes place according to the laws of the undulatory theory: and there can hardly be a doubt that dispersion in all other bodies will be found to follow the same law. It is, however, an express condition of the connexion between the velocity of light and the length of its undulations, that the intervals between the vibrating molecules of the ethereal fluid should bear a sensible relation to the length of an undulation. The coincidence of the computed with the observed refractions shows that this condition is fulfilled within the refracting media; but the aberration of the fixed stars leads to the inference that it does not hold in the ethereal regions, where the velocities of the rays of all colours are the same. Strong as all that precedes is in favour of the undulatory theory, the relative velocity of light in air and water is the final and decisive proof. By the Newtonian theory the velocity is greater in water than in air, by the undulatory theory it is less; hence if a comparison could be made it would decide which is the law of nature. The difficulty consisted in comparing the velocity of light passing through a small extent of water with the velocity of light in air, which is 10,000 times greater than the velocity of the earth in its orbit. This delicate and difficult experiment was made by means of an instrument invented by Professor Wheatstone for measuring the velocity of electricity. It consists of a small mirror which revolves in its own plane like a coin spinning on its edge. When it revolves very rapidly the reflected image of an object changes its place perceptibly in an inconceivably small fraction of a second. The mirrors used in the experiment were made to revolve more than 1000 times in a second, by which means the places of the two images—one from light passing through air, and the other from light passing through an equal length of water-were found to be such as to prove that the velocity of light in air and in water is as 4 to 3, while by the Newtonian theory it is as 3 to 4. By this final and decisive proof the undulatory theory may from henceforth be regarded as the law of nature. This experiment was accomplished by M. Fizeau and M. Léon-Faucault, at the suggestion of M. Arago, whose eyesight did not permit him to undertake it himself.
Chemical or Photographic Rays of Solar Spectrum-Scheele, Ritter, and Wollaston's Discoveries - Wedgwood's and Sir Humphry Davy's Photographic Pictures - The Calotype — The Daguerreotype- The Chromatype The Cyanotype-Collodion Sir John Herschel's Discoveries in the Chemical Spectrum - M. Becquerel's Discoveries of Inactive Lines in ditto Thermic Spectrum - Phosphoric Spectrum- Electrical Properties Parathermic Rays— Moser and Hunt's Experiments - General Structure and antagonist Properties of Solar Spectrum - Defracted Spectrum.
THE Solar Spectrum exercises an energetic action on matter, producing the most wonderful and mysterious changes on the organised and unorganised creation.
All bodies are probably affected by light, but it acts with greatest energy on such as are of weak chemical affinity, imparting properties to them which they did not possess before. Collodion and metallic salts, especially those of silver, whose molecules are held together by an unstable equilibrium, are of all bodies the most susceptible of its influence; the effects, however, vary with the substances employed, and with the different rays of the solar spectrum, the chemical properties of which are by no means alike. As early as 1772 M. Scheele showed that the pure white colour of chloride of silver was rapidly darkened by the blue rays of the solar spectrum, while the ́red rays had no effect upon it and in 1801 M. Ritter discovered that invisible rays beyond the violet extremity have the property of blackening argentine salts, that this property diminishes towards the less refrangible part of the spectrum, and that the red rays have an opposite quality, that of restoring the blackened salt of silver to its original purity; from which he inferred that the most refrangible extremity of the spectrum has an oxygenising power, and the other that of deoxygenating. Dr. Wollaston found that gum guaiacum acquires a green colour in the violet and blue rays, and resumes its original tint in the red. No attempt had been made to trace natural objects by means of light reflected from them, till Mr. Wedgwood, together with Sir Humphry
Davy, took up the subject: they produced profiles and tracings of objects on surfaces prepared with nitrate and chloride of silver, but they did not succeed in rendering their pictures permanent. This difficulty was overcome in 1814 by M. Niepcé, who produced a permanent picture of surrounding objects by placing in the focus of a camera-obscura a metallic plate covered with a film of asphalt dissolved in oil of lavender.
Mr. Fox Talbot, without any knowledge of M. Niepce's experiments, had been engaged in the same pursuit, and must be regarded as an independent inventor of photography, one of the most beautiful arts of modern times: he was the first who succeeded in using paper chemically prepared for receiving impressions from natural objects; and he also discovered a method of fixing permanently the impressions-that is, of rendering the paper insensible to any further action of light. In the calotype, one of Mr. Talbot's applications of the art, the photographic surface is prepared by washing smooth writing-paper, first with a solution of nitrate of silver, then with bromide of potassium, and again with nitrate of silver, drying it at a fire after each washing; the paper is thus rendered so sensitive to light that even the passage of a thin cloud is perceptible on it, consequently it must be prepared by candle-light. Portraits, buildings, insects, leaves of plants—in short, every object is accurately delineated in a few seconds; and in the focus of a camera-obscura the most minute objects are so exactly depicted that the microscope reveals new beauties.
Since the effect of the chemical agency of light is to destroy the affinity between the salt and the silver, Mr. Talbot found that, in order to render these impressions permanent on paper, it was only necessary to wash it with salt and water, or with a solution of iodide of potassium. For these liquids the liquid hyposulphites have been advantageously substituted, which are the most efficacious in dissolving and removing the unchanged salt, leaving the reduced silver on the paper. The calotype picture is negative, that is, the lights and shadows are the reverse of what they are in nature, and the right-hand side in nature is the left in the picture; but if it be placed with its face pressed against photographic paper, between a board and a plate of glass, and exposed to the sun a short time, a positive and direct picture, as it is in nature, is formed: engravings may be exactly copied
by this simple process, and a direct picture may be produced at once by using photographic paper already made brown by exposure to light.
While Mr. Fox Talbot was engaged in these very elegant discoveries in England, M. Daguerre had brought to perfection and made public that admirable process by which he has compelled Nature permanently to engrave her own works; and thus the talents of France and England have been combined in bringing to perfection this useful art. Copper, plated with silver, was successfully employed by M. Daguerre for copying nature by the agency of light. The surface of the plate is converted into an iodide of silver, by placing it horizontally with its face downwards in a covered box, in the bottom of which there is a small quantity of iodine which evaporates spontaneously. In three or four minutes the surface acquires a yellow tint, and then, screening it carefully from light, it must be placed in the focus of a camera obscura, where an invisible image of external objects will be impressed on it in a few minutes. When taken out, the plate must be exposed in another box to the action of mercurial vapour, which attaches itself to those parts of the plate which had been exposed to light, but does not adhere to such parts as had been in shadow; and as the quantity of mercury over the other parts is in exact proportion to the degree of illumination, the shading of the picture is perfect. The image is fixed, first by removing the iodine from the plate by plunging it into hyposulphite of soda, and then washing it in distilled water; by this process the yellow colour is destroyed, and in order to render the mercury permanent, the plate must be exposed a few minutes to nitric vapour, then placed in nitric acid containing copper or silver in solution at a temperature of 614° of Fahrenheit for a short time, and lastly polished with chalk. This final part of the process is due to Dr. Berre, of Vienna.
Nothing can be more beautiful than the shading of these chiarOscuro pictures when objects are at rest, but the least motion destroys the effect; the method therefore is more applicable to buildings than landscape. Colour is wanting; but the researches of Sir John Herschel give reason to believe that even this will ultimately be attained.
The most perfect impressions of seaweeds, leaves of plants
feathers, &c., may be formed by bringing the object into close contact with a sheet of photographic paper, between a board and plate of glass; then exposing the whole to the sun for a short time, and afterwards fixing it by the process described. The colours of the pictures vary with the preparation of the paper, by which almost any tint may be produced.
In the chromatype, a peculiar photograph discovered by Mr. Hunt, chromate of copper is used, on which a dark brown negative image is first formed, but by the continued action of light it is changed to a positive yellow picture on a white ground; the farther effect of light is checked by washing the picture in pure water.
In cyanotypes, a class of photographs discovered by Sir John Herschel, in which cyanogen in its combinations with iron forms the ground, the pictures are Prussian blue and white. In the chrysotype of the same eminent philosopher, the image is first received on paper prepared with the ammonia-citrate of iron, and afterwards washed with a neutral solution of gold. It is fixed by water acidulated with sulphuric acid, and lastly by hydriodate of potash, from which a white and purple photograph results. It is vain to attempt to describe the various beautiful effects which Sir John Herschel obtained from chemical compounds, and from the juices of plants; the juice of the red poppy gives a positive bluish purple image, that of the ten-week stock a fine rose colour on a pale straw-coloured ground.
Pictures may be made by exposure to sunshine, on all compound substances having a weak chemical affinity; but the image is often invisible, as in the Daguerreotype, till brought out by washing in some chemical preparation. Water is frequently sufficient; indeed Sir John Herschel brought out dormant photographs by breathing on them, and some substances are insensible to the action of light till moistened, as for example, gum guaiacum. Argentine papers, however, are little subject to the influence of moisture. The power of the solar rays is augmented in certain cases by placing a plate of glass in close contact over the sensitive surface.
All these various experiments, though highly interesting, have now been superseded. It was found that paper did not always answer for photography, on account of imperfections in its structure; silver plates were too expensive; and glass was