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