These

Lerebours, well known as one of the most distinguished opticians in Paris, has collected more than twelve hundred Daguerreotype views of the most beautiful scenery and antiquities in the world. The remarkable views from the East were taken by MM. Horace Vernet and Goupil. M. Las Cases has furnished the interesting scenery of St Helena; and M. Jomard has been occupied with Spanish scenery and the beauties of the Alhambra. Daguerreotype pictures, of which it is impossible to speak too highly, are engraved in aqua tinta, upon steel, by the first artists; and they actually give us the real representation of the different scenes and monuments at a particular instant of time, and under the existing lights of the sun and the atmosphere. The artists who took them, sketched separately the groups of persons, &c., that stood in the street, as the Daguerreotype process was not then sufficiently sensitive to do this of itself; but in all the landscapes, which shall now be reproduced by this singular art, we shall possess accurate portraits of every living and moving object within the field of the picture.*

It would be almost an insult to our readers to dwell with any detail on the utility of the new art, in promoting and extending science. We have already seen its advantages in giving the most faithful representation of objects of natural history; and it cannot fail to be equally useful in all the sciences of observation, where visible forms are to be represented. The civil engineer and the architect have claimed it as an art incalculably useful in their profession; and the meteorologist has seized upon it as a means of registering successive observations of the barometer, thermometer, hygrometer, and magnetometer, in the observer's absence; and thus exhibiting to his eye, at the end of every day, accurate measures of all the atmospheric changes which have taken place. We shall not say any thing at present of the great discoveries to which it has already conducted us in physical

* M. Lerebours' work is entitled Excursions Daguerriennes, collection de 50 planches, representant les Vues et les Monumens les plus remarquable du Globe. The views are from Paris, Milan, Venice, Florence, Rome, Naples, Switzerland, Germany, London, Malta, Egypt, Damascus, St Jean D'Acre, Constantinople, Athens, &c.

†This application will be understood by supposing a sheet of sensitive paper to be placed behind the mercurial column of the barometer, and a light before the same column: the shadow of the top of the mercury will leave a white image on the paper blackened by the light, and the paper itself being moved behind the mercury by a clock, we shall thus observe the various heights of the mercury depicted at every instant of time.

optics, as we must devote a separate part of this article to their discussion.

In thus stating the peculiar advantages of Photography, we have supposed the Daguerreotype and Calotype to be the same art. Our readers have already seen in what the difference really consists; but it is still necessary that we should attempt to draw a comparison between them, as sister arts, with advantages peculiar to each.

In doing this, our friends in Paris must not suppose that we have any intention of making the least deduction from the merits of M. Daguerre, or the beauty of his invention; which cannot be affected by the subsequent discovery of the Calotype by Mr Talbot. While a Daguerreotype picture is much more sharp and accurate in its details than a Calotype, the latter possesses the advantage of giving a greater breadth and massiveness to its landscapes and portraits. In the one, we can detect hidden details by the application of the microscope; in the other, every attempt to magnify its details is injurious to the general effect. In point of expense, a Daguerreotype picture vastly exceeds a Calotype one of the same size. With its silver plate and glass covering, a quarto plate must cost five or six shillings, while a Calotype one will not cost as many pence. In point of portability, permanence, and facility of examination, the Calotype picture possesses a peculiar advantage. It has been stated, but we know not the authority, that Daguerreotype pictures have been effaced before they reached the East Indies; but if this be true, we have no doubt that a remedy will soon be found for the defect. The great and unquestionable superiority of the Calotype pictures, however, is their power of multiplication. One Daguerreotype cannot be copied from another; and the person whose portrait is desired, must sit for every copy that he wishes. When a pleasing picture is obtained, another of the same character cannot be produced. In the Calotype, on the contrary, we can take any number of pictures, within reasonable limits, from a negative; and a whole circle of friends can procure, for a mere trifle, a copy of a successful and pleasing portrait. In the Daguerreotype the landscapes are all reverted, whereas in the Calotype the drawing is exactly conformable to nature. This objection can of course be removed, either by admitting the rays into the camera after reflection from a mirror, or by total reflection from a prism; but in both these cases, the additional reflections and refractions are accompanied with a loss of light, and also with a diminution, to a certain extent, of distinctness in the image. The Daguerreotype may be considered as having nearly attained perfection, both in the

quickness of its operations and in the minute perfection of its pictures; whereas the Calotype is yet in its infancy-ready to make a new advance when a proper paper, or other ground, has been discovered, and when such a change has been made in its chemical processes as shall yield a better colour, and a softer distribution of the colouring material.

In the preceding pages we have treated of the history, the processes, the advantages, and the relative merits of the Daguerreotype and the Calotype, considered as two existing arts which we owe to M. Daguerre and Mr Talbot; and, under this restriction, we have not felt ourselves called upon to give any particular account of the experiments and improvements of Dr Fyfe, M. Claudet, Mr Hunt, Mr Ponton, M. Lassaigne, M. Netto, and many other writers. The necessary restriction of our limits, indeed, renders it impossible to enter into those minute details and discussions, which, though they might be less acceptable to a general reader, could not fail to be extremely interesting to those who may be engaged in the practice of these fascinating arts. The same cause has prevented us from describing the construction and use of the different camera-obscuras, with lenses and mirrors, which have been, or which may be, successfully employed in Photography.

Extensive, however, as the subject is, and restricted as we are, there are three philosophers, Sir John Herschel, Dr Draper of New York, and Professor Moser of Konigsberg, who have applied the photographic processes with such distinguished success to the advancement of optical science, that it would be unpardonable to withhold from our scientific readers an account of their discoveries; even had they been less important and of a less popular character than they are.

*

The researches of Sir John Herschel were both practical and theoretical. In the first portion of the paper which contains them, he treats of the various parts of the photographic processes; and in the second, he treats of the chemical and calorific action of the solar rays. In the very important process of fixing photographs, whether negative or positive, Sir John gives the preference to the hyposulphite of soda.t The photograph is first

*On the Chemical Action of the Rays of the Solar Spectrum on Preparations of Silver and other Substances, both Metallic and NonMetallic, and on some Photographic Processes. By Sir John F. W. Herschel, Bart., K.H. V.P.R.S.,' Phil. Trans. 1840; pp. 1-60.

The use of ammonia for fixing positive photographs was tried, but

well washed by soaking in water. When thoroughly dried, it is then brushed over very quickly with a flat camel-hair brush, dipped in a saturated solution of the hyposulphite, first on the face, then on the back. When the picture has been thus completely penetrated by the fluid, it must be washed repeatedly and copiously with water, until the water comes off without the slightest sweetness. Sir John recommends the repetition of this process, especially if the paper be thick. The use of common salt he has never found satisfactory; and though he regards the hydriodate of potash as good for fixation, if the right strength be hit, yet in the case of negative photographs its use would be injurious, from the yellow tint which it gives to the ground of the picture. In using a weak solution of corrosive sublimate, Sir John 'discovered a very singular effect of it. When the picture was washed over with this solution, and then laid for a few minutes. in water, the picture was completely obliterated. But though invisible, it was only dormant, for it could easily be revived, in all its force, by merely brushing it over with a solution of a neutral hyposulphite. In this way it may be successively obliterated and revived as often as we please.

The numberless combinations' of chemical substances which were tried by Sir John Herschel, with the view of increasing the sensitiveness and facility of preparation of photographic paper, did not lead him to any very satisfactory results; and with the candour which distinguishes him, he most 'readily admits that the specimens (of photographic paper) recently placed in his hands by Mr Talbot, far surpass, in point of sensitiveness, any that he had yet produced of a manageable kind.' Following Mr Talbot's principle of successive alternate washes with salt and nitrate of silver, Sir John adopted the following series of washes, viz. :—

1. Nitrate of silver. Spec. grav. 1.096, (say 1.1). 2. Muriate of soda. 1 salt, 19 water. 3. Nitrate of silver. Spec. grav. 1.132, (say 1.15). saturating the muriatic solution with chloride of silver, and occasionally dividing the last, or third, application into two conse

abandoned by Mr Talbot. Mr Constable of Jesus College, Cambridge, afterwards found it to be efficacious; and we have ourselves found it to be preferable to any other fixing liquid. When applied copiously and repeatedly, the photographs will resist the direct and continued light of the sun. As the ammonia always weakens the picture, the positive photographs should be strongly brought out by the sun. When they are weak, the bromide of potassium is preferable as the fixing material.

cutive washes of nitrate of silver, of equal strength, by dilution. As an ordinary working paper easily prepared, Sir John considers it as having sensibility enough for most purposes. It gives, he says, good camera pictures, and when smooth demy paper is used, it retains its whiteness even in the dark. As all other papers suffered discoloration under the preceding process, and as the smooth demy might not always be obtained of the same quality, Sir John was induced to adopt, for camera pictures, a process which proved both convenient and effectual; and which he found to apply equally well to both descriptions of paper-that is, the blue wove post and smooth demy. He simply delays the last or 'efficient wash of nitrate of silver, on which the sensitive quality depends, till the moment of using it; and, in fact, using the paper actually wet with the nitrate, and applied with its sensitive face against a glass plate, whose hinder surface is in the focus of the camera. This affords other collateral advantages: • 1st, That all crumpling or undulation of the paper is avoided; 2d, That being rendered in some degree transparent, the light is enabled to act deeper within its substance.'

6

6

In the practice of Photography, the artist is often disturbed with imperfections in his paper, even when it has been prepared with the utmost attention. Both Mr Talbot and Sir John Herschel have paid particular attention to this imperfection; and have, we have no doubt, ascertained the general cause of these spots, as well as a probable means of preventing them.

'I will now add,' says Mr Talbot, a few remarks concerning the very singular circumstance which I have before briefly mentioned-viz., that the paper sometimes, although intended to be prepared of the most sensitive quality, turns out on trial to be wholly insensible to light, and incapable of change. The most singular part of this is the very small difference in the mode of preparation, which causes so wide a discrepancy in the result. For instance, a sheet of paper is all prepared at the same time, and with the intention of giving it as much uniformity as possible; and yet, when exposed to sunshine, this paper will exhibit large white spots of very definite outline, where the preparing process has failed; the rest of the paper, where it has succeeded, turning black as rapidly as possible. Sometimes the spots are of a pale tint of cerulean blue, and are surrounded by exceedingly definite outlines of perfect whiteness, contrasting very much with the blackness of the part immediately succeeding. With regard to the theory of this, I am only prepared to state as my opinion at present, that it is a case of what is called "unstable equilibrium." The process followed is such as to produce one of two definite chemical compounds; and when we happen to come near the limit which separates the two cases, it depends upon exceedingly small and often imperceptible circumstances, which of the two compounds shall be formed. That they