PHOTOGRAPHY is both an art and a science. As an art it enables us to draw, depict, or write by means of light. As a science it teaches us how to observe and to investigate the effects produced by light upon all natural bodies, whether animate or inanimate, mineral, vege table, or animal Its study as an art is of comparatively recent date, but the science had previously excited the attention of nearly all the most eminent investigators in modern science. The names of Davy, Wedgwood, Thomas Young, Wollaston, and the two Herschels in this country—of Scheele, Ritter, Seebeck, Berthollet, and Becquerel on the Continent—testify to this effect. Photography is worthy of special attention from the fact that it requires for its rational and thoroughly successful pursuit a knowledge of chemistry, optics, and physics gene rally, together with an amount of artistic taste and manual dexterity, such as must be useful not only for purposes of mental training, but under a variety of circumstances in actual life. The variety of its parts and aims gives it a special charm for those who like to have a pursuit admitting of both activity of mind and body ; its processes are as much carried on out of doors as in close laboratories. Further it has this charm, that while it furnishes problems of the greatest interest and intricacy for the most advanced philosopher in optics or chemistry, it has its practical processes, which may be readily appre hended, and exercised for purposes of utility or recreation by those who are but little skilled in physical manipulations.
The history of photography has been so fully treated of by Mr. Robert Hunt, in his ' Researches on Light,' and in his ' Treatise on Photography,' and also by the Abbd Moigno, in his ' Repertoire d'Op tique Moderne,' that wo need not do here more than recapitulate in a brief manner the points of chief interest which they have given at greater length.
It may be well to say at the outset, that it was not till the year 1839 that Photography acquired for itself distinct recognition, through the investigations of Fox Talbot and Daguerre, which resulted in the introduction of the two processes known as the Calotype or Talbotype, and Daguerreotype. As usual in the history of art and science,
approximations had been attained to by earlier experimentalists. It is interesting to inquire into the labours of some of these. Proceeding historically, we shall find that observations relating to the science of photography precede the first attempts at establishing the principles of the art.
In 1722 Petit noticed that solutions of nitrate of potash and muriate of ammonia crystallised more readily in the light than they did in darkness. In 1777 Scheele wrote, "It is well known that the solution of silver in acid of nitre, poured on a piece of chalk and exposed to the beams of the sun, grows black. The light of the sun reflected from a white wall has the same effect, but more slowly, heat without light being without effect." Again, "Fix a glass prism at the window, and let the refracted sunbeams fall on the floor. In this coloured light put a paper strewed with tuna cornea (chloride of silver), and you will observe that this horn silver grows sooner black in the violet ray than in any of the other rays." Senebier repeated these experiments, and also experimented on the influence of light in the bleaching of wax.
In 1793 Count Rumford sent to the' Philosophical Transactions' a memoir entitled 'An Inquiry concerning the Chemical Properties that have been attributed to Light.' In this paper the Count attempts to prove that all the effects produced upon metallic solutions by bright sunshine are due to heat. In 1802 Mr. Harrup refuted this view, and showed that several salts of mercury were reduced by light alone, and not by heat.
In 1801 Ritter. ,proved the existence of rays in the solar spectrum, which are to be found beyond its visible limits, and that these rays have the power of darkening chloride of silver. These researches having excited attention, Mid. Berard, Seebeck, Berthollet, Sir W. Herschel, Sir H. Englefield, Wollaston, Davy, and others, made various experiments which tended still further to confirm the proof that light had a special influence over bodies beyond that exercised through its heat ; and that the colour of the light was in some way related to this newly observed action of the sunbeam.