It is doubtful whether any industrial art has made such rapid strides within the last thirty years as that of Photography. Founded upon the simple discovery that a certain chemical salt—the chloride of silver—becomes blackened upon exposure to light, the art has grown step by step into an important national industry. It would be next to impossible to estimate the number of persons who, directly and indirectly, owe their daily bread to King Sol in his character of Artist. A glance at the advertisement columns of one of the journals devoted to this interest will give us some idea of the busy number of camp-followers running in the wake of the huge army of photographic artists of Great Britain alone. Opticians, paper-makers, chemical manufacturers, glass-makers, cabinet-makers, besides a host of others who supply the et-cÆteras of the business, vie with each other in the adaptability of their goods. Other countries can no doubt shew a similar list—notably France, whose paper is used by photographers throughout the world.

Although the peculiar affinity of silver chloride for light was discovered by Scheele just one hundred years ago, its application to art was not recognised until the year 1839, when Daguerre in France and Talbot in England almost simultaneously hit upon the method of rendering permanent the pictures which had been before obtained, but which had faded away into darkness as quickly as the daylight which had given them birth. This discovery of fixing the image, as it is technically called, was really the starting-point of an art, samples of which, good, bad, and indifferent, are now to be found in every homestead in the kingdom.

The mysterious power which could seize almost instantaneously the fleeting appearances of moving life, could not fail to take a strong hold on the public attention. Other art-pursuits had of course previously had numerous aspirants, but they came and went as fashions do, without leaving any permanent good behind them. Not so photography, which is perhaps unique in owing its present state of perfection to the exertions and patient investigations of mere amateurs. The reason of this unusual state of things is probably due to the fact that photography has required a large expenditure of both time and money to bring it to maturity; both which commodities are more plentiful with those who have not to work for daily bread.

The earliest sun-pictures, as produced by Daguerre, and named after him, were formed on silver plates treated with iodine. After exposure in the camera, they were developed by the action of mercury vapour, which attached itself to those portions of the plate which had received the greatest amount of light. Such pictures were necessarily difficult of multiplication, each impression requiring a separate exposure and development. Examples of this early method of photography may still be seen in many houses, where they have been carefully treasured as mementoes of friends who have passed away. These pictures are by no means of a permanent nature, the action of the air contributing with other causes to tarnish the silver plate, and so gradually to destroy the image thereon.

The discovery of the collodion process by Archer in 1851, quite supplanted the previous method, and gave photography an impetus which has carried it rapidly forward to the present date. Numerous substances have been tried at different times to support mechanically the delicate sun-printed image, but nothing has as yet been found to equal collodion upon glass.

Photographic art has now become such a thing of our every-day life, that perhaps there is scarcely an intelligent person who does not know the difference between a negative and a positive. Every one nowadays has his or her portrait taken at least once, and can well remember the nervousness incidental to a first visit to the photographic studio. Usually the photographer is kind enough to allow his anxious client a glimpse of the picture in its earliest stage, when the lights are where the shades ought to be, and vice versÂ. Such is the negative, from which any number of positives may be printed by the action of sunlight on prepared paper placed underneath it. These silver prints (for although the silver plate is banished with the old method, chloride of silver contained in the pores of the paper still holds its own) have unfortunately the character of not being as permanent as they might be. This fault is commonly attributed to carelessness in not thoroughly eliminating the salt used in fixing the pictures; so that, by a strange anomaly, the discovery which claimed to make our photographs permanent is now charged with the sin of causing their ultimate deterioration. Photographers complain that the great competition, which has led to the adoption of low-priced work, will not permit them to give to the washing of the prints the time and attention which permanence demands. There are no doubt other causes at work in our heavily charged town atmospheres which have a destructive effect on our photographs. At anyrate, be the cause what it may, it is the rule and not the exception to find a paper print of, say ten years old, sadly faded and generally disfigured. Such a great disadvantage as this has met with an antidote in the shape of a discovery which has to a certain extent superseded the practice of silver printing. We allude to the carbon process, which is dependent upon the curious fact that bichromatised gelatine, after exposure to light, becomes insoluble. That is to say, a mixture of gelatine with the bichromate of an alkali—such as the bichromate of potash—will remain soluble so long as it is excluded from light. Carbon in the form of lampblack, or indeed any pigment, is mixed with this bichromatised gelatine, and paper coated therewith is exposed under a negative in the same way as in the case of a silver print, warm water being afterwards used to wash out those portions of the prepared surface which the sunlight has not rendered immovable.

Such, briefly, is the mode of producing the so-called carbon pictures, which without doubt are, as they claim to be, as lasting as the paper on which they are printed. They are not equal, in point of brilliancy, to the better known silver pictures, but this disadvantage is more than counterbalanced by their good keeping qualities. The word carbon as here used is a misnomer, for as we have already indicated, other pigments, most of which have a metallic origin, may be used in the process.

Photography as now practised may be classed under two general heads—the wet process and the dry process: the first being solely dependent upon the use of collodion and the silver bath; the other dispensing with either or both. Hitherto, the great obstacle to the landscape photographer has been the cumbrous nature of the impedimenta necessary to the production of pictures at a distance from home. It is by no means an easy matter to transport a dark tent containing a chemical laboratory, together with a camera and the necessary supply of water, from one place to another. Moreover, the scenes which naturally tempt the artist lie in unfrequented, and oftentimes in almost inaccessible places. The use of dry plates, by which the necessity of a tent is altogether obviated, has rendered the art far more easy of accomplishment, and has thus placed outdoor photography amongst those pastimes which a non-professional can successfully pursue. In the wet process the sensitive collodion plate must be exposed to the air within a very few minutes of its removal from the silver bath, otherwise it becomes quite useless; the object of the various dry processes being to preserve the film in a sensitive state, so that it can be exposed as occasion may require, and developed in the studio at a future time. It is needless to point out that this method of photography dispenses at once with any travelling gear except the camera and lens, and a convenient light-tight receptacle for the sensitive plates. Many ingenious contrivances are now used in the form of changing boxes—as they are called—by which plates may easily be transferred to the camera without danger of exposing them to any accidental gleam of light. The jealousy with which a tourist naturally guards his treasured dry plates has more than once roused the suspicions of the acute Custom-house officer, who, in his zeal for the welfare of the revenue, has unwittingly spoilt the produce of many days' careful work, by insisting upon opening the strange-looking box!

Although it would be beyond the scope of this paper to enter into detailed explanation of the manner in which dry plates are prepared, the importance of the subject must claim some attention at our hands. In order to render a collodion plate capable of being kept indefinitely in a dried and sensitive condition, it is found that a solution of some organic substance must be washed over it, and dried with it. To enumerate all the various agents that have been employed for this purpose, would be impossible. Tea, coffee, sugar, tannin, gum, gelatine, with many other compounds, have each found favour with different experimenters, and with varying success; but the last-named substance, gelatine, is perhaps likely to supersede all the others, as giving more satisfactory and constant results. Plates thus prepared, although almost wholly disregarded by the professional artist, have, on account of their portable nature, a large sale among the amateur members of the photographic world. They are also almost exclusively used in astronomical photography, a branch of the art to which we will now direct the reader's attention.

It will be remembered that on the occasion of the last eclipse of the sun, expeditions to observe it were sent out from nearly every country of the civilised world; each expedition depending largely upon photography as a means of recording its labours. Although the state of the weather at many of the selected stations rendered the apparatus useless, a great number of pictures were actually obtained, a comparison of which set at rest certain theories relating to appearances which had up to this time been the subject of much discussion and speculation. No human hand could have correctly depicted such an ever-varying object as the sun presented at this time, to say nothing of the well-known fact that the power of correctly estimating appearances varies so much with individuals, that a comparison of mere drawings would be quite useless for the purpose in view. The cause of the periodical changes in the sun's spots yet remains to be discovered; and it is probable that the photographs which are being almost hourly taken (having for their object the solution of this problem) will ultimately lead to a satisfactory result.

The transit of Venus represents another important field of inquiry in which photography has done useful work. The expeditions fitted out two years ago, with their splendid array of modern instruments, would compare strangely with the preparations for the investigation of 1761, when Captain Cook started on his ill-fated voyage to Otaheite. Still more vivid does the progress of scientific research become when we remember that the very first observation of the transit of Venus was made one hundred years earlier, with no better apparatus than a bit of smoked glass. When we consider that the main value of such an observation rests upon the appearances recorded at the moments of ingress and egress of the planet upon the sun's face, the importance of a means for securing instantaneous pictures will be appreciated. It is true that certain optical defects exist in these pictures which prevent their use for the purpose of reliable measurement; but these obstacles, we trust, may be overcome by 1882, when the next transit will be due.

The practice of micro-photography—that is, a combination of the camera with the microscope—has lately met with some attention among scientific men, and there are now many workers who are trying to bring it into the prominence which it deserves. Formerly, drawings of microscopic preparations could only be secured by means of a prism (or camera lucida, as it is called), fitted on to the eyepiece of the microscope, by which means an enlarged spectral image of the object became apparent on a sheet of paper placed near the instrument; the lines thus exhibited being rendered serviceable by the careful use of the lead-pencil. It is obvious that such means afforded a very imperfect representation of the image as it really appeared in the field of the microscope, even if the operator possessed some amount of artistic skill; but now, by the aid of the camera, a picture of the most unfailing accuracy can be secured in a fraction of a second. Such rapidity is only required, however, where the object is of a fluid or animated nature, as in the case of moving organisms. We venture to think that there is a great future in store for micro-photography.

One of the most recent applications of photography to scientific uses is exemplified in its adaptation to the spectroscope, by which we are furnished with evidence of the composition of the heavenly bodies. Any account of this marvellous device we must, however, leave for a future paper. In the fine and useful arts, photography now plays an important part. Portraits, life size, executed in oil, are successfully painted from small photographic likenesses, at a comparatively small cost; and with this important advantage, that the likeness in every case is unchallengeable. This may be considered a great triumph in the photographic art.

This power of enlargement to any reasonable dimensions is a great addition to the resources of the photographer; and it is not alone confined to portraiture, as the numerous large-sized landscapes constantly exhibited will testify. In former times, when the lenses then in use were capable of including but a small portion of a view, the only way to secure large pictures was to take them in sections, and afterwards to join the paper prints. The lines of junction were naturally a great disfigurement to the finished result, to say nothing of the extra labour which such mode of proceeding involved. The impossibility of preserving the exact tone of colour in these different sections through all the vicissitudes of printing, toning, and fixing, was also enough to condemn the process. These difficulties have been altogether obviated by the construction of lenses which will include any amount of the view before which they are placed, and which moreover give a picture so perfect in detail as to admit of being greatly magnified without injury to its beauty. The enlargement is now carried out by a copying camera of the form of the well-known magic lantern, and lighted by an oxy-hydrogen or magnesium burner. The negative takes the place of the ordinary painted slide, and the enlarged image is projected upon a sensitive surface.

Perhaps the greatest problem which the photographer has to solve is the production of landscapes with their natural canopy of clouds. This difficulty will be understood when we explain that the sky being such a brilliant object, requires but a very small fraction of the exposure which is demanded by the grass and trees beneath it. The plan generally adopted is to secure a separate negative for each of these component parts of the picture, and to join them mechanically previous to the operation of printing. The beautiful instantaneous marine studies which we all admire—and which represent the clouds in every variety of form—are produced without this double exposure; for it is obvious that the reflective property of water confers equal brightness on all parts of the view.

The production of photographic pictures in printing-ink by means of the press is now receiving a great deal of attention. Most of the processes adopted owe their origin to the effective mixture of gelatine and bichromate of potash. It will be necessary to explain that the gelatine so treated is not only—after exposure to light—rendered insoluble, but it becomes quite non-absorbent of water. This property is taken advantage of in the following manner. A thick plate of glass or metal coated with the mixture is exposed under a negative, and afterwards placed for a time in cold water. It is then found that those parts of the plate which represent the lights of the picture remain flat; whilst the other portions which have been protected from the light swell up into high-relief. The plate can then be rolled with ordinary printing-ink, and impressions taken to any reasonable amount.

Space will not permit us to detail the various modifications of this process which exist under different designations. Metal plates can now, by a very similar treatment, be made ready for the etching acid. Wood-blocks which no artist but the sun has touched, can be given to the engraver ready to his hand. The lithographic printer is also independent of the draughtsman, for absolutely perfect fac-similes of maps, plans, &c.; line-subjects can also be produced in endless quantity.

The applications of this wonderful art are already legion, and are so continually receiving additions, that we may hope that its sphere of usefulness will be extended beyond all present calculation. As a means of livelihood for thousands, its importance in a commercial sense is invaluable, while as the handmaid of the philosopher, it fulfils a higher duty, in helping us by sure and certain steps to the attainment of scientific truth.