NOTHING further appears to have been done in regard to photography until Thomas Wedgwood, a son of the famous potter, took up the subject, and succeeded in producing a photograph by making use of Scheele’s observations on chloride of silver. He was, however, unable to fix his pictures, which blackened and disappeared as soon as they were exposed to diffused light. He was assisted in these experiments by Humphrey Davy, then just rising into fame as a chemist, and in 1802, after Wedgwood’s death, Davy prepared an account of the experiments they had made, and the results obtained, in the journal of the Royal Institution for 1802. In this paper the process they adopted is described as ” a method of copying paintings upon glass, and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy.”
Wedgwood placed flat bodies, such as the pinatifid leaves of plants, upon paper that had been prepared with nitrate of silver. By this means, when exposed, the light was kept from passing through the paper covered by the objects. Those parts remained white, whilst the uncovered portions of the paper were blackened by the light. Hence there was produced a white outline of the superimposed objects upon a black ground.
Another experiment which Wedgwood and Davy tried was to place a solar microscope in the aperture of a camera obscura in order, if possible, to fix on sensitized paper the image produced on the screen. They fastened a sheet of paper saturated with a silver salt at the spot where the image fell, and left it there for several hoursthough without result.
As these experiments in sun-drawing are of great importance in the history of photography, the following extracts from the paper published in the journal describing the experiments of the joint investigators will be of interest, as being the first published account of the production of photographic pictures.
” White paper, or white leather, moistened with a solution of nitrate of silver, undergoes no change when kept in the dark ; but on being exposed to the daylight it speedily changes colour, and after passing through different shades of grey and brown, becomes at length nearly black. The alterations of colour take place more speedily in proportion as the light is more intense. In the direct beam of the sun, two or three minutes are sufficient to produce the full effect ; in the shade several hours are required ; and light transmitted through different coloured glasses act with different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little effect upon it. Yellow and green are more effective, but violet or blue produce the most powerful or decided effect,
“When the shadow of any figure is thrown upon the prepared surface, the part concealed by it remains white, and the other parts speedily become dark. For copying paintings on glass the solution should be applied on leather ; and in this case it is more readily acted on than when paper is used. After the colour has been once fixed on the leather or paper, it cannot be removed by the application of water, or water and soap, and it is in a high degree permanent. The copy of a painting or a profile, immediately after being taken, must be kept in an obscure place ; it may, indeed, be examined in the shade, but in this case the exposure should only be for a few minutes; by the light of candles or lamps, as commonly employed, it is not sensibly affected. No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by light have as yet been successful. They have been covered by a thin coating of fine varnish, but this has not destroyed their susceptibility of becoming coloured; and even after repeated wash ings, sufficient of the active part of the saline matter will adhere to the white parts of the leather or paper to cause them to become dark when exposed to the rays of the sun. Besides the applications of this method of copying that have just been mentioned, there are many others; and it will be useful for making delineations of all such objects as are possessed of a texture partly opaque and partly transparent. The woody fibres of leaves, and the wings of insects, may be pretty accurately represented by means of it ; and in this case it is only necessary to cause the direct solar light to pass through them, and to receive the shadows upon leather.
The images formed by means of a camera obscura have been found to be too faint to produce, in any moderate time, an effect upon the nitrate of silver. To copy these images was the first object of Mr. Wedgwood in his researches on the subject, and for this purpose he first used nitrate of silver, which was mentioned to him by a friend as a substance very sensible to the influence of light ; but all his numerous experiments as to their primary end proved unsuccessful. In following these processes, I have found that the images of small objects, produced by means of the solar microscope, may be copied without difficulty on prepared paper. This will probably be a useful application of the method ; that it may be employed successfully, however, it is necessary that the paper be placed at but a small distance from the lens.”
In this record it is stated that the experiments went to prove that muriate of silver was more readily acted upon by the light than the nitrate.
Here we have most of the elements of the great discovery, the chief thing lacking being the power to fix or render permanent the images. Some years later (1801-11) Seebeck carried these discoveries as to the coloration of chloride of silver still further. He showed that the violet rays turn it brown, the blue producing a shade of blue, the yellow preserving it white, and the red constantly giving a red shade to the salt.
In order to explain their various effects properly it will be necessary to enter a little more fully into the composition of light.
A beam of light is composed of a bundle of rays, a ray being the smallest portion of light that can emanate from a luminous body. Every one of such rays possesses distinctive qualities, both as regards their chemical functions and their colours. Sir Isaac Newton demonstrated that the white light emitted from the sun is composed of rays of different colours and tints. The way in which he made plain this fact was by means of a prism, which is the name given to a triangular piece of glass used in optics, so placed in a darkened room as to receive upon one of its surfaces a ray of light from a small aperture bored in the window-shutter.
Before the prism is fixed in position the sunbeam entering the aperture at A will fall upon the white screen at E. But as soon as the prism B C (Fig. 8) is fixed in the path of the sunbeam, thus enabling it to fall on the side B, the ray will be refracted, or bent out of its course, and pass to the back of the prism (as in the line D), instead of along the line A E, which it would otherwise have taken if the prism had not been interposed.
This, however, is not all: for in place of the simple round spot of light which fell upon the screen at E prior to the interposition of the prism, we see that an elongated, delicately-coloured image has been formed at , D. If we now stand a short distance from the prism it will be seen that these colours are spread out in a triangular form, the base of which is on the screen, and the apex, or point of origin, at the back (C) of the prism.
This coloured image is known as the prismatic or solar spectrum, which, according to Newton’s theory is composed of seven different colours (Fig. 9). The colour at the lower portion of this image, or that nearest to the round white spot at E on the screen, when the prism was away, is of a red colour, and the one at the opposite end is violet the intermediate space being occupied by five other colours, indigo, blue, green, yellow, and orange, coming in rotation between the violet and the red. Or, as the poet puts it :
First the flaming red Sprung vivid forth : the tawny orange next ; And next delicious yellow : by whose side Fell the kind beams of all-refreshing green, Then the pale blue, that swells autumnal skies, Eternal played ; and then, of sadder hue, Emerged the deepened indigo, as when The heavy-skirted evening droops with frost, While the last gleamings of refracted light Died in the fainting violet away.
Newton was of opinion that these seven were all primary colours, and that each was possessed of a certain degree of refrangibility. Later investigations, however, have tended to reduce the number of colours to six, namely, red, orange, yellow, green, blue and violet ; and despite Brewster’s contention that there are only three primary colours, red, yellow, and blue, the others being composed of the overlapping of these, it is now generally held that each of the six colours above named are primary.
As we have seen, since Newton’s time researches into the nature of the spectrum have been greatly extended. From experiments made in 1812, M. Berard demonstrated that chemical intensity is greatest at the violet end of the spectrum, and that it extends, as Ritter and Wollaston had previously noticed, a little beyond that extremity. He concentrated by means of a lens all that part of a spectrum comprised between the green and the red, and by means of another lens, all that portion between the green and the violet. By this experiment he found that the chloride of silver, placed at the focus of the first bundle of rays, underwent no modification after an exposure of two hours, while that which was placed under the focus of the second group, which was much less bright and less hot, blackened in a very few minutes.
It has been found that the upper or invisible violet end of the spectrum can be made visible by allowing it to fall upon some suitable fluorescent substance, such as uranium glass, or a solution of quinine. Indeed, by properly choosing the sensitive substance, photographs can be taken of any part of the spectrum, from a reduction of nearly four times the wave length of the red rays up to the highest chemical rays known to exist.
Upon these experiments was based the theory, which subsequent investigations have fully borne out, that there exist three spectrums one above the other, namely a calorific or heat spectrum, a colorific, and a chemical spectrum. It is further established that each of the substances which compose the three spectrums, and even each molecule of unequal refrangibility which constitutes these substances, is endowed, like the molecules of visible light, with the property of being polarized by reflection, and of escaping from reflection in the same positions as the luminous molecules.
Since the above experiments were made a great deal of attention has been paid to this subject, and our knowledge of the properties of the spectrum has been greatly advanced. It will suffice here, however, to say that the ultra violet rays perform certain chemical actions, and make various substances shine in the dark. The infra-red rays, on the other hand, have no chemical properties, and quench luminosity. What they do is to warm the substances on which they fall, and hence are called calorific waves. Maxwell in 1867 propounded the brilliant theory that all light waves, visible and invisible, are really electric vibrations, and recent discoveries have proved the truth of the hypothesis.
Of the real nature of the light rays, beyond this generalisation, little is known. Various theories have been advanced respecting them; but into these it is not necessary for our present purpose to enter. What concerns us more nearly is the fact that a sunbeamthe ray of white lightcontains within it powers of which the earlier philosophers had but a faint idea. Besides its accompanying heat, it has the power of decomposing and determining the recomposition of chemical components. This principleknown as actinismis as perfectly distinct in the nature of its properties from light, as light is from the principle of heat, with which it is so closely connected.
Actinism is regarded by some as the fundamental principle upon which photography is based. That principle is a component part of light, but not light itself. In explanation of this seeming anomaly it may be explained that, of the prismatic spectrum, the violet end possesses the greatest reducing or decomposing power, that is, the violet part of the decomposed portion of light, and still more strongly the invisible rays beyond it, exerts the most powerful influence upon the unstable metallic salts, reducing them speedily to their bases.
What may be called the varying actinic powers of the different colours can be demonstrated by a very simple experiment. Take a piece of sensitized paper, and place on it three pieces of coloured glass, red, yellow and blue. Expose the whole to the sun’s rays for a short time, and it will be found that the paper has become rapidly discoloured under the blue glass, but remains unchanged under the red and yellow, although the last is by far the most transparent. It is this property possessed by red and yellow of intercepting the actinic rays of light that makes them so useful in practical photography for the construction of the “darkroom.”