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[ March, 10-2 THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. twelve months from their formation, have, nevertheless, somewhat altered during the second year. This circumstance, added to the fact that the first attempts which I made became indistinct in process of time (the paper growing wholly dark), induced me to watch the progress of the change during some considerable time, as I thought that perhaps all these images would ultimately be found to fade away. I found, however, to my satisfac tion, that this was not the case ; and, having now kept a number of these drawings during nearly five years without their suffering any deterioration, I think my self Authorised to draw conclusions from my experiments with more certainty. 2. Effect and Appearance of these Images.—The images obtained in this manner are themselves white, but the ground upon which they display them selves is variously and pleasingly coloured. Such is the variety of which the process is capable, that by merely vary ing the proportions, and some trifling details of manipulation, any of the following colours are readily attainable:—Sky-blue, yellow, rose-colour, brown of various shades, and black. Green alone is absent from the list, with the exception of a dark shade of it, approaching to black. The blue- coloured variety has a very pleasing effect, somewhat like that produced by the Wedgwood-ware, which has white figures on a blue ground. This variety also retains its colours perfectly if preserved in a portfolio, and not being subject to any spontaneous change requires no preserving process. These different shades of colour are, of course, so many different chemical compounds, which chemists have not hitherto distinctly noticed. 3. First Applications of this Process.—The first kind of objects which I attempted to copy by this process were flowers and leaves, either fresh, or selected from my herbarium. These it renders with the utmost truth and fidelity, exhibiting even the venation of the leaves, the minute hairs that clothe the plant, &c. &c. It is so natural to .associate the idea of labour with great complexity and elaborate detail of execution, that one is more struck at seeing the thousand florets of an Ayrostis depicted with all its capillary branchlets (and so accu rately that none of all this multitude shall want its little bivalve calyx, re quiring to be examined through a lens), than one is by the picture of the large and simple leaf of an oak or a chesnut. But, in truth, the difficulty is in both cases the same. The one of these takes no more time to execute than the other; for the object which would take the most skilful artist days or weeks of labour to trace or to copy, is effected by the boundless powers of natural chemistry in the space of a few seconds. To give an idea of the degree of accuracy with which some objects can be imitated by this process, I need only mention one instance. Upon one occa sion, having made an image of a piece of lace of an elaborate pattern, I showed it to some persons at the distance of a few feet, with the inquiry whether it was a good representation ? when the reply was, “ That they were not so easily to be deceived, for that it was evidently no picture, but the piece of lace itself.” At the very commencement of my experiments upon this subject, when I saw how beautiful were the images which were thus produced by the action of light, I regretted the more that they were destined to have such a brief existence, and I resolved to attempt to point out, if possible, some method of preventing this, or retarding it as much as possible. The following consi derations led me to conceive the possibility of discovering a preservative process:— The nitrate of silver, which has become black by the action of light, is no longer the same chemical substance that it was before. Consequently, if a picture produced by solar light is subjected afterwards to any chemical pro cess, the white and dark parts of it will be differently acted upon ; and there is no evidence that, after this action has taken place, these white and dark paits will any longer be subject t» a spontaneous change; or, if they are so, still it does not follow that that change will now tend to assimilate them to each other. In cases of their remaining dissimilar the pic ure will remain visible, and therefore our object will be accomplished. If it should be asserted that exposure to sunshine would necessarily reduce the whole to one uniform tint, and destroy the picture, the onus probandi evidently lies on those who make the assertion. If we designate by the letter A the exposure to the solar light, and by B some indeterminate chemical process, my argument was this: —Since it cannot be shown, d priori, that the final result of the series of processes ABA will be the same with that denoted by B A, it will be, therefore, worth while to put the matter to the test of experiment, viz., by varying the process B until the right one be dis covered, or until so many trials have been made as to preclude all reasonable hope of its existence. My first trials were unsuccessful, as, indeed, I expected; but after some time I discovered a method which answers perfectly, and shortly afterwards another. On one of these more especially I have made numerous experi ments; the other I have comparatively little used, because it appears to re quire more nicety in the management. It is, however, equal, if not superior, to the first in brilliancy of effect. This chemical change, which I call the preserving piocess, is far more effectual than could have been anticipated. The paper, which had previ ously been so sensitive to light, becomes completely insensible to it, inso much that I am able to show the society specimens which have been exposed for an hour to the full summer sun, and from which exposure the image has suffered nothing, but retains its perfect whiteness. 4. On the Art of fixing a Shadow.—The phenomenon which I have now briefly mentioned appears to me to paitake of the character of the marvellous, almost as much as any fact which pltysical investigation has yet brought to our knowledge. The most transitory of things—a shadow, the most pro verbial emblem of all that is fleeting and momentary—may be fettered by the spells of our “ natural magic,” and may be fixed for ever in the position which it seemed only destined for a single instant to occupy. This remarkable phenomenon, of whatever value it may turn out in its application to the arts, will, at least, be accepted as a new proof of the value of the inductive methods of modern science, which by noticing the occur rence of unusual circumstances (which accident, perhaps, first manifests in some small degree), and by following them up with experiments, and varying the conditions of these until the true law of nature which they express is apprehended, conducts us at length to consequences altogether unexpected, remote from usual experience, and contrary to almost universal belief. Such is the fact, that we may receive on paper the fleeting shadow, arrest it there, and in the space of a single minute fix it there so firmly as to be no more capable of change, even if thrown back into the sunbean from which it derived its origin. 5. Before going further I may, however, add, that it is not always neces sary to use a preserving process. This I did not discover until after I had acquired considerable practice in this art, having supposed at first that all these pictures would ultimately become indistinct if not preserved in some way from the change. But experienco has shown to me that there are at least two or three different ways in which the process may be conducted, so that the images shall possess a character of durability, provided they are kept from the action of direct sunshine. These ways have presented themselves to notice rather accidentally than otherwise ; in some instances without any particular memoranda having been made at the time ; so that 1 am not yet prepared to state accurately on what particular thing this sort of semi-dura bility depends, or what course is best to be followed in order to obtain it. But as I have found that certain of the images which have been subjected to no preserving process remain quite white and perfect after the lapse of a year or two, and, indeed, show no symptom whatever of changing, while others differently prepared (and left unpreserved) have grown quite dark in one tenth of that time, I think this singularity requires to be pointed out. Whether it will be of much value I do not know. Perhaps it will be thought better to incur at first the small additional trouble of employing the preserving pro cess, especially as the drawings thus prepared will stand the sunshine ; while the unpreserved ones, however well they last in a portfolio or in common daylight, should not be risked in a very strong light, as they would be liable to change thereby even years after their original formation. This very quality, however, admits of useful application. For this semi-durable paper, which retains its whiteness for years in the shade, and yet suffers a change whenever exposed to the solar light, is evidently well suited to the use of a naturalist travelling in a distant country, who may wish to keep some memo rial of the plants he finds, without having the trouble of drying them and carrying them about with him. He would only have to take a sheet of this paper, throw the image upon it, and replace it in his portfolio. The defect of this particular paper is, that in general the ground is not even ; but this is of no consequence where utility alone, and not beauty of effect, is consulted. 6. Portraits.— Another purpose for which I think my method will be found very convenient, is the making of outline portraits, or silhouettes. These are now often traced by the hand from shadows projected by a candle. But the hand is liable to err from the true outline, and a very small deviation causes a notable diminution in the resemblance. I believe this manual pro cess cannot be compared with the truth and fidelity with which the portrait is given by means of solar light. 7. Paintings on Glass.—The shadow-pictures which are formed by ex posing paintings on glass to solar light are very pleasing. The glass itself, around the painting, should be blackened; such, for instance, as are often employed for the magic lantern. The paintings on the glass should have no bright yellows or reds, for these stop the violet rays of light, which are the only effective ones. The pictures thus formed resemble the productions of the artist’s pencil more, perhaps, than any of the others. Persons to whom I have shown them have generally mistaken them for such, at the same time observing that the style was new to them, and mustbe one rather difficult to acquire. It is in these pictures only that, as yet, I have observed indications of colour. I have not had time to pursue this branch of the inquiry further. It would be a great thing if by any means we could accomplish the delinea tion of objects in their natural colours. I am not very sanguine respecting the possibility of thi3; yet, as I have just now remarked, it appears possible to obtain at least some indication of variety of tint. 8. Application to the Microscope—I now come to a branch of the subject which appears to me very important, and likely to prove extensively useful, the application of my method of delineating objects to the solar microscope. The objects which the microscope unfolds to our view, curious and wonder ful as they are, are often singularly complicated. The e)e, indeed, may comprehend the whole which is presented to it in the field of view; but the powers of the pencil fail to express these minutiae of nature in their innumera ble details. What artist could have skill or patience enough to copy them? Or granting that he could do so, must it not be at the expense of much most valuable time, which might be more usefully employed? Contemplating the beautiful picture which the solar microscope produces, the thought struck me whether it might not be possible to cause that imago to impress itself upon the paper, and thus to let Nature substitute her own inimitable pencil for the imperfect, tedious and almost hopeless attempt of copy ing a subject so intricate ? My first attempt had n<? success, Although I chose a bright day, and formed