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century. It was not until the year 1802 that the chemical properties of light were made available for the production of pictures or images of objects, and even then to a very slight extent. Sir Humphrey Davy and Mr. Wedgewood discovered a preparation by means of which writingpaper could be rendered sensitive to the action of strong light, thus seeming to tread upon the verge of very important discoveries; but the results of these experiments were so unsatisfactory, both as regarded the quality of the pictures produced and their incapability of resisting the action of light afterwards, that eventually they relinquished the project.

Among the numbers who from this time were employed in investigating the properties of light, Mons. Niepce, a Frenchman, appears to have made the next advance in the photographic art. He commenced his researches about the year 1814, and not until the lapse of thirteen years was he prepared to show any results at all proportioned to the duration and extent of his inquiries, when he forwarded to the Royal Institution of London (1827) a paper upon the subject of his discovery, which he there named “Heliography,” signifying drawn by the sun, with some specimens, on metal plates, of the art. He had, previously to this time, accidentally made the acquaintance of a Mons. Daguerre, who was employed in similar investigations, and in the year 1829 they decided to execute a deed of copartnership relating to their past and future discoveries. Still at this time the delineations obtained were exceedingly imperfect. Ten to fourteen hours were necessary for their production, which, from the motion and change of shadows caused by the sun's rays in that time, with other causes, rendered them at best very dubious representations.

Another ten years elapsed, during which time M. Niepce died, and M. Daguerre, still persevering in his efforts, continued to improve the process so greatly that, in the year 1839, the French government, after investigating its merits, secured to him, by legal act, a pension of six thonsand francs, and to M. Isidore Niepce, son of the deceased M. Niepce, also a pension of four thousand francs, in consideration of which they were bound to make known to the world the entire process, as well as any future improvements in it which they might at any time make.

It must not be supposed that during all these years English philosophers were paying no attention to the subject of Photography. In the year 1834 a Mr. Talbot was succeeding in the production of some very beautiful pictures upon paper, previously rendered sensitive to luminous influence by means of the nitrate of silver, or common caustic. By the agency of the solar microscope he was enabled to execute, in the full sunshine, these copies of objects in the space of half a second, which process he communicated without reserve to the Royal Society six months before the publication of Daguerre's process in France. He has since patented an improved modification of his discovery, under the name of the “ Calotype," which, in point of efficiency, is the nearest in approach to the Daguerreotype of the many and varied processes which have of late been introduced to the world by Sir John Herschell, Mr. Hunt, and others ; such, for example, as those termed Cyanotype, Anthotype, Energiatype, &c.

Immediately upon the publication of M. Daguerre's secret in France, whose name henceforth became connected with the art which forms the subject of this paper, the English patent law permitted the right of its exclusive practice to an applicant in this country; and thousands of very beautiful specimens of the art have been executed here, notwithstanding

the enormous price, as compared with the cost, which this monopoly secures to the patentee.

The composition of the plates upon which these pictures are executed is copper plated with silver, in the proportion of about one part of the latter to twenty-five of the former. The quantity of silver used however, varies much with different makers. The plates undergo a careful and longcontinued planishing from the maker's hands, until a true and highly polished surface is obtained. The operator has now to commence a cleaning process, in which the following substances may be used : pure tripoli, fine ground emery, prepared rottenstone ; either of which are applied to the plate upon fine cotton-wool, with dilute nitric acid or spirits of wine. To convey an idea of the nicety and cleanliness required in these operations, it may be remarked that, if the slightest grease or dust be present in any of these preparations, or in the cotton-wool, no picture, or an imperfect one, will be obtained, when placed in the camera afterwards. For this reason a piece of cotton-wool once used in rubbing the plate is necessarily thrown aside ; for contact either with the fingers or the plate renders it sufficiently soiled to hinder rather than facilitate the progress of the operation. The plate must next be heated over a spirit-lamp to evaporate the mercury used in its composition, again polished with cotton-wool and tripoli, and finally with prepared lampblack upon the velvet buffer. It now possesses the finest surface it is capable of receiving, and is ready for the next stage of the process. This is subjecting it to the vapour of iodine in a closed box for a few minutes, until it has obtained a fine yellow colour, caused by a combination of the iodine and silver, and forming a very thin film of the iodide of silver. The light must be carefully excluded from it during this and all subsequent operations; for a momentary exposure, as will be seen hereafter, would be injurious to the effect. The plate is now in that condition of sensitiveness that any image thrown upon it, by means of a lens or reflector, in a good light, would be permanently received in about five minutes. This, however, in most cases of landscape taking, would be inadequate to fix those transcient and momentary effects of a gleam of sunshine or passing cloud, the faithful delineation of which renders a Daguerreotyped picture peculiarly beautiful and inimitable. It would also interfere considerably with the easy and comfortable position of the sitter, when the art is applied to portraiture; as it must be evident that the slightest motion in any object during the operation of copying, would produce a double image, and of course destroy the truth of the picture. Many attempts have been made to increase the sensitiveness of a plate in the condition just described, viz. that of being simply iodized, and many accelerating substances have been recommended by their respective inventors. That which perhaps is now as much in use as any is a preparation containing bromine in solution, and its application forms the next step in the process. The plate is removed over a cup or glass containing this liquid, where it continues from thirty to fifty seconds, according to the strength of the mixture, which varies by evaporation, and also yields the principle in vapour more rapidly in proportion as the temperature of the surrounding atmosphere is increased. The only accurate mode of testing it is to examine the colour of the plate, which may be done by a hasty glance, with the assistance of reflected light, direct rays being too powerful: when its surface is of a clear rose tint, the operation is complete. Were the plate to be exposed to the vapour either of the iodine or brominc for an indefinite period, the colour would rapidly

pass from yellow to rose, and from rose to violet, in which condition the power of exhibiting photographic images would be almost extinct.

The plate thus prepared is ready for the operation which converts its surface into a picture. It has been hitherto scrupulously excluded from the light; it must now be placed in such a position that the image of the object it is required to copy shall be clearly defined upon its face, either by reflection from a concave mirror, or by refraction through the medium of a lens. This is accomplished by placing it in a small case while in the darkened room in which the preceding processes have been effected, so that it may be protected from the action of light, and thus removed to the camera or reflector. The case is so constructed that the side which covers the polished surface of the plate falls at the will of the operator, and exposes it to receive an impression from the outlines which are pencilled by the rays of light upon it. The period thus occupied, and consequently that during which the sitter is required to remain immoveable, varies somewhat with the state of the atmosphere and the time of day. About thirty to forty seconds may be considered a fair average sitting. The case is here now immediately closed; for not a glimpse of light may disturb the plate until it has undergone an operation for the purpose of rendering visible the effects of the previous one. Were we to examine it now, we should see nothing but the bright rose-coloured surface, slightly tinged in places with violet; and this, while we gazed, would soon become one mass of deep purple—not a trace of the beautiful picture, already in embryo, should we be able to discover. It is therefore transferred to the mercury-box, and subjected to the action of that metal upon its surface, in the form of vapour caused by the application of gentle heat, while it occupies an oblique position above the mercury, forming an angle of forty-five degrees with the horizon. This process occupies from fifteen to twenty-five minutes, and during this time it must be watched in order to ascertain the effects produced. Apertures containing yellow glass are provided in the box for this purpose, through which, by the light of a taper, a glance at the plate may be obtained. It may be remarked here that glass of this description, admitting only the yellow or orange ray, prevents injurious results to the picture, as the chemical action of light resides principally in the violet ray, and is scarcely perceptible in the former: but of this hereafter.

The effect of this last operation has been to render visible the whole picture, and thus to show to the operator the result of his labours. It presents rather a cadaverous appearance, and is a faithful copy of the individual who sat to the magic painter. If he laughed, or knit his brows which last is most usually the case, from the difficulty of supporting a steady gaze under any considerable degree of light, or if he has engaged an awkward position expressly for the occasion, an effect not at all unlikely to follow, from an extreme desire to be easy and look well, there it is fixed—made faithfully prominent in the miniature, which perchance may be designed for very tender and sentimental uses. Fidelity from the Daguerreotype we may expect, but not a spark of flattery.

It now only remains to set the picture; that is, to protect it from the further action of light, which has been partially but not wholly arrested by its coating of mercury. An account of this process will more properly follow after the rationale of that already described has been furnished; and, in order to effect this, it will be necessary to pay some attention to the properties of light.

The agency of light being evidently that which produces the process we have already detailed, it will be necessary that its composition and mode of action (although to some extent our acquaintance with these is but conjectural) should become subjects of examination. Let it be understood that it is not our province now to enter into the merits of any theories which may have been propounded respecting light, but simply to state its known properties in conventional language, which, although not perhaps strictly accordant with a true idea of its action, will convey more intelligible notions of the subject than any other. For example, when speaking of a ray of light, we shall understand by that term an emanation of luminous and other influences from the sun, regardless of an hypothesis which considers light to be a property of matter excited by means of some unknown solar influence, or of any other, excepting the popular one, which education and sensual impressions have led us to follow.

In what follows, we therefore speak positively, not conjecturally. The first and most evident property of that which we term light, is luminous influence, or the cause of objects around us becoming cognizable to our visual organs, and which may be spoken of as luminosity. The second is also evident; its power of communicating or producing heat. The third, is its power of exciting chemical action in some bodies, and of disturbing their molecular arrangement; that is, causing a different arrangement in the minute and invisible atoms of which all substances in nature are composed. For instance, a sunbeam cannot pass over any form of matter, organic or inorganic, without leaving traces of its path. True, these may not be appreciable by the naked eye, but they nevertheless exist, and are to be made manifest by an application of the proper test. What a field for interesting inquiry does not the knowledge of such a fact present! What an idea does it afford of the magnitude and marvellous multiplicity of those influences which the action of light is daily exerting over the varied elements which compose our globe and its inhabitants!

This chemical power of light has been recently termed actinism ; and actino-chemistry distinguishes that class of chemical phenomena which is excited by its influence. The working of this principle we shall observe in in the process under our notice.

From exposure of the polished surface of the silver-plate to the vapour of iodine, a thin film of the ioduret of silver is formed upon it. This is slightly changed by its subsequent exposure to the solution of bromine, as previously described, becoming now the bromo-iodide of silver. This substance is changed by the action of light, and rendered capable of receiving and retaining vapour in proportion to the intensity of the rays which have fallen upon it. The precise manner in which this change is effected is still a matter of doubt. But we may venture thus far to speculate: The bromo-iodide of silver upon the plate's surface has been formed, it will be recollected, in the dark: no light has as yet fallen upon it. Were a ray to be admitted through yellow glass, or any transparent yellow medium, still there would be no change. There would be abundance of luminosity in this instance, but no chemical action proving to us that these two influences, luminosity and actinic power, although both dwelling in the same ray, are separable. Now let us suppose an admission of pure light: The prepared surface is rapidly changing; it may be, assuming a chrystalline structure; at all events, it is so altered in character that, although but the moment previously it had no capacity for the retention of vapour, it now

possesses it in an extensive degree. Now once more suppose the light to have been transmitted through a purple or violet medium, the change would still have taken place, although in this instance the condition of luminosity could not co-exist, proving that the change was caused by the actinic power, which resides principally in the violet ray, and entirely independent of luminosity. The other property of light, viz. the power of communicating heat, also forms no part of the agency in effecting this change. This it is equally easy to separate from its adjunct properties; but no detail of experiments can be necessary in order to prove that heat alone, any more than mere luminous influence, is not the agent in producing pictures by the photographic process.

We shall now suppose the plate removed to the mercury-box. It was stated that its surface had been rendered capable of retaining vapour, in proportion to the intensity of the rays which have acted upon it. In the case of a portrait, for example, we may imagine the sitter's dress to have exhibited the extremes of white and black, while the flesh-colour of the face and hands furnished middle tints. His image, reflected or refracted, as the case may be, was composed of various rays, proceeding from his person, of different powers. The white sends forth a full strong ray, which rapidly acted upon the sensitive coating; while the black, absorbing every ray, and reflecting none, produced no effect upon the corresponding portion of the plate; and every intermediate tint was proportionably marked accordingly as it partook more of light or shadow. During its exposure to the mercurial vapour, those parts unaffected by the light, for instance the black coat or hair, remain in their original state; that is, in a condition in which vapour will not adhere, and consequently present to the eye the lustrous black surface of the naked silver: while those parts which received the rays of light, becoming readily coated by the minute globules of mercury, which are in this form white, present light tints answering to those furnished by the person of the sitter. This mode of action of course holds good whether the denomination of picture be landscape or architectural, and applies to the practice of either equally well. .

The remaining manipulations which the picture undergoes have for their object the removal of superfluous iodine, or of any impurities which may have attached to it during the operation, and consist chiefly in washing it with distilled water and in a solution of the hyposulphite of soda. An additional beauty is given by heating the plate over a spirit-lamp, its surface being at the same time covered with a liquid preparation of the chloride of gold, which imparts a warm tinge to the whole, resulting from a combination of the gold with the mercurial deposit. This process accomplishes all that is necessary to the complete finish of the picture. Yet, astonishing as it must appear, it may still undergo another, which converts it into an engraved copper-plate, adequate to the printing of a number of impressions upon paper, and this by the aid, among other things, of electrical action. What an illustration is this of the truth of an allusion made at the outset to the extent of man's dominion over the great powers which nature employs in her operations ! That our very pictures, and these too of the best and most truthful character, should be thus underwritten when correctly described, “Painted by Light,” “Engraved by Electricity."

There are few subjects which have of late yielded so rich a harvest of discoveries to the inquirer as those connected with the operations of light; and in the course of time, as the result of further research, more confident

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