Although, in order of date, the investigations of Sir John Herschel and others have a priority over those particular experiments of Mr. Talbot's which resulted in the discovery of his very beautiful process, the calotype, yet to avoid confusion, it is thought advisable to group together the discoveries of each investigator, where this is practicable, in our historical division.
The earliest productions of Mr. Talbot were simply such preparations as those already described, in which a chloride of silver was formed on the surface of the paper, with some nitrate of silver in excess. These need not be any further described than they have already been.
Early in 1840, drawings on paper were handed about in the scientific circles of London and of Paris, which were a great advance upon anything which had been previously done. These were the results of a new process discovered by Mr. Talbot, and then attracted so much attention, that M. Biot made them the subject of a communication to the Academy of Sciences in Paris. His remarks are printed in extenso in the Comptes Rendus, from which the following passages are translated, as they bear particularly on many of the defects which still continue to prove annoyances in the photographic process to which they have reference.
Many of the remarks have a peculiar value from the suggestions they contain, and they are worthy of record as marking the period when the French were first made acquainted with the processes on paper, as practised in England. Some disposition has been shown on the part of several continental photographers to claim originality for processes published in England many years before their own were devised, and which singularly resemble them. After remarking that many very important physical facts were being developed by the study of photography, M. Biot continues :—
" It is not to be expected that photogenic drawings, made on paper, can ever equal the clearness and fineness of those obtained on level and polished metallic plates. The texture of paper, its superficial roughnesses, the depth of the imbibitions, and the capillary communication established between the various unequally marked parts of its surface, are so many obstacles to absolute strictness of delineation, as well as to the regular gradation of tints in the camera obscura ; and the influence of these obstacles is greater when the chemical operation is slowly carried on. But when there is no pretence or necessity for submitting to the delicacies of art—when it is required, for example, to copy rare manuscripts faithfully—if we have papers which are very susceptible of receiving impressions in the camera obscura, they will suffice perfectly ; particularly when they present, like those of Mr Talbot, the facility of immediately procuring copies of the primitive drawing. It will, therefore, doubtless be found more commodious, and often even more practicable, to put four or five hundred drawings in a portfolio, than to carry about a similar provision of metallic plates with those indispensable protectors, squares of glass, to cover them. Attempts are being made, at this time, to fix the images produced by the Daguerreotype—perfect prints, it is true, but which are as light as the vapour from which they are produced; and, indeed, to bring a voluminous collection of these fragile products through the accidents incident to long, and sometimes perilous voyages, is a task requiring no ordinary care. But whoever has attentively studied the combination of physical conditions whence these admirable images result, will find it very difficult—I am far from saying impossible—to fix them, without destroying, or at least without essentially altering, the causes which produce their charm ; and then, for the purposes which I have mentioned, papers very susceptible of impression would still have the advantages of being less troublesome in removal from place to place, as also of more easy preservation.
" The utility of sensitive papers for copying texts was a natural consequence of the clearness of the copies of engravings which Mr. Talbot had already obtained by application, and which were presented to the Academy. He has included others among those just sent : there are also added specimens of this especial application, consisting of copies of a Hebrew psalm, of a Persian Gazette, and of an old Latin chart of the year 1279. Our brethren of the Académie des Belles Lettres, to whom I exhibited these impressions, were pleased to remark the fidelity of the characters, and their clearness, by which they are rendered as legible as the original text. Doubtless an old manuscript may be copied more quickly and more accurately by this means than by hand, even when the language in which it is written is understood. However, we must stop here. These copies are obtained by application : we must be enabled to obtain them by immediate radiation in the camera obscura. It is the only means of extending the process to papyrus and other opaque manuscripts, or which are not sufficiently transparent for radiation to traverse them. Moreover, the application of leaves is very difficult when they are bound up in a volume, and cannot be detached from one another.
"But this important extension will require much physical perfecting, towards which experimenters should direct their efforts. The first thing will be to augment the sensibility of the paper as much as possible, in order that the capillary communication of its various parts may not have sufficient time to deteriorate the effects of the local and immediate action of the radiation. I should be led to believe that it is principally to this kind of communication should be attributed the fact remarked by Mr. Talbot, that, in experiments by application, it is more difficult to copy clearly a tissue of black lace spread on a white ground, than white lace on a black ground; two cases of which he here gives examples. But another more hidden and more general difficulty seems to me to proceed from the unequal faculty of various substances for reflecting the radiations which strike them, and perhaps from their aptitude for making them undergo physical modifications. For example, you wish to copy by radiation in the camera obscura a picture painted on canvas, wood, or porcelain: the different colouring substances employed by the painter are placed and distributed in such a manner that each of them absorbs certain portions of the total incidental light, and reflects especially towards your eye the complementary portions, wherein predominate the rays proper to form the tint of which it would give you the sensation. But the chemically active reagent which the same parts of the picture receive and reflect, is distinct from the light which affects your retina. In order that the chemical effect which it produces on the sensible paper, or on M. Daguerre's layer of iodine, may present, in light or in shade, the equivalent of the coloured parts, it is requisite —1st, that this reflected radiation be chemically active; 2d, that the energy of its action be proportional to the intensity of illumination operated in the eye by the portion of luminous radiation reflected from the same point of the picture. Now this latter concordance certainly should not be fulfilled in an equal degree, by the various colourng matters, which affect the eye in the same manner, and which the painter may substitute for one another in his work. Substances of the same tint may present, in the quantity, or the nature of the invisible radiations which they reflect, as many diversities, or diversities of the same order, as substances of a different tint present relative to light: inversely they may be similar in their property of reflecting chemical radiations, when they are dissimilar to the eye: so that the differences of tint which they presented in the picture made for the eye, will disappear in the chemical picture, and will be confused in it in a shade, or of an uniform whiteness. These are the difficulties generally inherent in the formation of chemical pictures; and they show, I think, evidently, the illusion of the experimenters who hope to reconcile, not only the intensity, but the tints of the chemical impressions produced by radiations, with the colour's of the objects from which these radiations emanate. However, the distant or near relations of these two species of phenomena are very curious to study, not only as regards the photogenic art, since that name has, very improperly, been given it, but likewise as regards experimental physics. I doubt not that examples of these peculiarities may be remarked in the images of natural objects and coloured pictures executed by the Daguerreotype; but very apparent ones may be seen among Mr. Talbot's present impressions. Thus, some of them represent white procelain vases, coloured shells, a candlestick (of metal) with its taper, a stand of white hyacinths. The whole of these objects are felt and perceived very well in their chemical image; but the parts which reflect the purely white light, probably also the radiations of every kind, are, relatively to the others, in an exaggerated proportion of illumination, which, it seems to me, must result, partially, from the capillary communication during the continuance of the action ; so that the inequality would be less if the paper were more sensitive or more rapidly acted on. In the hyacinth, the stalk and the green leaves have produced scarcely a faint trace of their configuration; but they are strongly defined, especially in the parts of the outline, where more or less perfect specular reflection takes place. The points of the candlestick (metallic), where this reflection occurred, are copied by white stains locally applied, and which deteriorate the effect of the whole by their disproportion. But this is seen especially in a picture by Correggio, the frame of which was very vividly copied, whilst the figure on the canvas was hardly perceptible. This disproportion of lustre in the reproduction of some white parts, especially when they are dull and consequently very radiating, is sensible in certain parts of views taken by Mr. Talbot, to the point of rendering difficult the interpretation of the object to which they belong. However, these views are very satisfactory, as being obtained on paper, in the present season. Moreover, by an advantage peculiar to the chemical preparation which Mr. Talbot uses, it appears that the operations once completed, the drawings are no longer alterable by radiation, even acting with much energy. Indeed, we have here, as an example, four proofs of the same view of Mr, Talbot's house, with an identical disposition of lights and shades ; so that some, at least, if not three out of four, must have been procured by superposition. Mr. Talbot is right in representing this property of reproduction as an especial advantage of his process, and it would indeed be very useful in voyages. I have exposed one of these drawings to the action of the sun, not very powerful, it is true, for several hours, and I have not perceived the slightest alteration in the lights. I think I understand that, in Mr. Talbot's opinion, the shades alone are strengthened under this influence. According to what I have just said, it should be expected that the triumph of this process, as of every other photogenic reproduction, would take place with objects of white and dull plaster. Indeed, Mr. Talbot's parcel contains eight copies of busts and statues; six of which chiefly, of various forms and sizes, present very remarkable results, especially taking into consideration the unfavourable season at which they were produced. Truly, there is not found in them the strict perfection of trace, nor the admirable gradations of lights and shades, which constitute the charm of M. Daguerre's impressions; and I again repeat it, that my expressions may not be exaggerated. But I also repeat, that representations on sensitive papers must be considered as principally applicable to a different object, which does not impose such strict conditions of art, requiring only faithful images, sufficiently clear in their details to be readily recognized, and which, moreover, being obtained with rapidity, by an easy manipulation, may be kept with very little care, comprised in great number in a small compass, and moved from place to place with facility. Mr. Talbot's papers already present many of these essential qualities, with the advantage of being able to furnish numerous copies immediately. His efforts, and those of others occupied with the same subject, will conclude by adding to them everything which may be desirable, provided that expectation, or the pretension of a perfection of art physically incompatible with operations on paper, do not give a false direction to their endeavours. However, not to appear to despair too much of the future, I may add that the height of success would consist in discovering a substance very susceptible of receiving impressions, which might be applied on a papyraceous leaf without penetrating deep into it, and which might, however, be fixed in it after the operation, as in Mr. Talbot's impressions. It does not seem necessary even that the first impression thus rapidly obtained should copy the lights and shades in their proper places, provided that its transparency and fixedness were such that we might deduce them from the application of copies wherein the inversion would be corrected. And perhaps, by this decomposition of the problem into two successive operations, one of the best ways is opened by which it may be resolved".