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INACTIVE SPACES IN THE SPECTRUM.

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extreme limit of chemical action; 3, the chemical spectrum on argentic bromide; 4, the Talbotype spectrum. Herschel, Hunt, and E. Becquerel have each succeeded more or less perfectly in obtaining coloured impressions of the spectrum upon argentic chloride, but they have been unable to fix them. Herschel (Phil. Trans. 1840, 19) says that the impression was found to be coloured with sombre but unequivocal tints, imitating those of the spectrum itself.' The coloration commenced in the orange rays. E. Becquerel appears to have obtained more brilliant colours by employing a plate of silver which had been superficially converted into subchloride by immersing it in diluted hydrochloric acid, and then making it the positive plate of a voltaic battery.

Inactive spaces occur in the chemical spectrum, which, as Becquerel and Draper have shown, correspond exactly with those which are found in the visible spectrum; but they extend also into the prolongation beyond the violet extremity, and occur there in great number.

These fixed lines may be obtained upon Talbotype paper or, still better, upon a surface of collodion, in the following manner :

FIG. IIO.

P

G

Let c, c (fig. 110), represent a camera which allows of a considerable range of adjustment; s is a small slit, admitting of adjustment, but usually presenting a width of about o'or inch (o.mm.25), through which a beam of solar or other light is either transmitted directly, or is reflected from a heliostat, or from a steel mirror; is a quartz lens with a focal length of from 15 to 30 inches (or from about 4 to 75 metre); p, a quartz prism, the axis of which is perpendicular to the axis of the crystal, and the refracting faces of which are worked so as to cut the optic axis of the crystal at equal angles. The lens and prism may indeed be made of glass free from striæ, but for accurate observation they must be constructed of rock crystal: the distance of the lens, 7, from the slit, s, is equal to twice its focal length. If the prism be placed so as to produce the minimum deviation of the ray, and as close to the lens as can be, a spectral

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LINES IN THE PHOTOGRAPHIC SPECTRUM.

[1279.

image of the aperture will be formed, and may be received upon a screen placed at as great a distance behind the lens, 7, as s is in front of it. All the coarser lines of the visible spectrum may be traced by the unaided eye, when the spectrum is received on a screen of ground glass: and if a sheet of turmeric paper, or a block of yellow uranium glass be used, many of the lines beyond the violet are also rendered visible: by substituting a sensitive surface, such as collodion, for the screen of white paper, a faithful copy of the more refrangible portion of these lines may be obtained.

(127 r) It is remarkable that the chemical rays are identical with those which produce fluorescence (110). If the solar rays be transmitted through a layer of a concentrated but colourless acidulated solution of quinine sulphate, or one of æsculin in ammonia, before they reach the photographic surface, but little extra-spectral prolongation of chemical action is produced upon the sensitive surface.

Again, if the solution be removed and the spectrum be received directly upon a screen of yellow uranium glass, or a card coated with a particular uranic phosphate (Stokes, Phil. Trans. 1862, 602), a visible prolongation of the chemical rays, crossed by dark lines of inactive spaces, is at once rendered visible.

By varying the source of light, the chemical powers of the spectrum are varied also. The chemical action of the flame of the hydrocarbons, however intense the light, is but feeble; that of the lime-light is much more marked, while that of the electric light between charcoal-points greatly surpasses either; and these results coincide exactly with the relative power of exciting the phenomena of fluorescence possessed by these different lights.

The chemical rays emitted by luminous objects vary greatly both in quantity and in quality, some sources of light emitting rays of much higher refrangibility than others. For example, the flame of ordinary coal-gas burned in admixture with air, so as to produce the blue light of a smokeless gas flame, gives out scarcely any rays capable of affecting an iodized collodion plate; whilst the same amount of gas burned in the ordinary manner for illumination, emits a very decided though limited amount of rays capable of producing chemical action. The rays emanating from the intensely hot jet of the oxyhydrogen flame, are nearly without action upon a sensitive surface of collodion; whilst if thrown upon a ball of lime, though it certainly is not hotter than the burning jet of gas, the light then emitted contains as large a proportion of chemical rays as the solar light, and of very nearly the same refrangibility. But the most remarkable source of the chemical rays is afforded by the light of the electric spark or of the voltaic arc, the chemical spectrum of which is four or five

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PHOTOGRAPHIC TRANSPARENCY.

271

times as long as the chemical spectrum obtainable from the sun itself.

(1278) Photographic Transparency of various Media.-Amongst the methods of testing the extent of chemical action of any given radiant source, the most convenient is that which is dependent upon the extent of photographic effect exerted upon a surface of collodion coated with argentic iodide, on which the spectrum is allowed to fall.

In no case does it appear that any non-luminous source can emit chemical rays of sufficient intensity to traverse ordinary refracting media; and amongst the rays given off by various luminous objects, it is found that the chemical effects upon the collodion plate are not perceptible in those portions upon which the first three-fourths of the visible spectrum has fallen, but they commence powerfully in the last fourth; and in the case of the electric spark are prolonged to an extent equal to between four and five times the length of the visible portion.

In the prosecution of some inquiries upon the photographic spectra of the metals in which the author was engaged, it was a desideratum to procure some substance which should possess a higher dispersive power than quartz, and which, whilst avoiding the double refraction of quartz, should yet allow the free passage of the chemical rays. He was hence led to try a variety of substances which, owing to their transparency to light, might reasonably be hoped to possess chemical transparency also for though it was known to those who have studied the spectrum, that many colourless substances besides glass exert an absorptive action upon some of these chemical rays, the subject had not at that time received the careful experimental examination which its importance warrants.

:

The inquiry soon extended itself beyond the limits originally proposed, and ultimately embraced a large number of bodies in the solid, liquid, and gaseous conditions (Phil. Trans. 1862, 861). These experiments showed that although rock-salt, fluor-spar, water, and some few other substances, are almost as diactinic (or chemically transparent, from dia, through, ȧkriv, a ray) as quartz, none of these bodies could be advantageously substituted for quartz in the construction of the prisms and lenses required in the investigations in which he was engaged.

Among the most remarkable results upon the photographic transparency of bodies are the following:

1. Colourless solids which are equally transparent to the visible rays, vary greatly in permeability to the chemical rays. 2. Bodies

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PHOTOGRAPHIC TRANSPARENCY.

[127 8.

which are photographically transparent in the solid form, preserve their transparency in the liquid and in the gaseous states. 3. Colourless transparent solids which absorb the photographic rays, preserve their absorptive action with greater or less intensity both in the liquid and in the gaseous states. 4. Pure water is photographically transparent, so that many compounds which cannot be obtained in the solid form sufficiently transparent for such experiments, may be subjected to trial in solution in

water.

The mode in which the experiments were conducted was the following:The source of light employed was the electric spark obtained between two metallic wires, generally of fine silver, shown at e, fig. 110, and connected with the terminals of the secondary wires of an induction coil, into the primary circuit of which was introduced a condenser, and into the secondary circuit a small Leyden jar. The light of the sparks was then allowed to fall upon the vertical slit, either before or after traversing a slice or stratum of the material, d, of which the photographic transparency was to be tested; the transmitted light was then passed through the quartz prism, placed at the angle of minimum deviation. Immediately behind this was the lens of rock crystal, and behind this at a suitable distance the spectrum was received upon the sensitive surface of collodion. The liquids under trial were contained in a small glass cell with quartz faces, forming a stratum 0'75 inch in thickness, which was traversed by the rays; gases and vapours were introduced into tubes two feet long closed at their extremities with thin plates of polished quartz. The following tables exhibit the relative diactinic power of a few of the various solids, liquids, and gases and vapours subjected to experiment as measured by the length of the relative spectra. In some cases the whole length of the spectrum showed evidence of partial absorption of the chemical rays, as the impression was enfeebled throughout its entire length.

The photographic image obtained upon the collodion plate commenced in each case at the same point of the spectrum, corresponding with a spot a little more refrangible than the line G. Calling the line H 100, and numbering backwards for the less refrangible rays, the line B being at 84, the commencement of the photograph in each case is at 96'5, and the extreme limit of the most refrangible rays 170'5.

Each photograph was obtained under circumstances varying only in the nature of the transparent medium through which the rays of the spark from the

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PHOTOGRAPHIC TRANSPARENCY.

273

silver points were made to pass, before they were allowed to fall upon the collodion plate. When absorption occurs, it is almost always exerted upon the most refrangible rays: but in the case of the coloured gases and vapours, chlorine, bromine, and iodine, the absorption differs from the general rule, and is by no means proportioned to the depth of colour. A column of chlorine with its yellowish-green colour cuts off the rays of the less refrangible extremity through fully two-thirds of the spectrum; the red vapour of bromine cuts off about one-sixth to the length of the spectrum, the absorbent action being limited to the less refrangible extremity; whilst the deep violet-coloured vapours of iodine allow the less refrangible rays to pass freely for the first fourth of the spectrum; then a considerable absorption occurs, and afterwards a feeble renewal of the photographic action is exhibited towards the more refrangible end.

In these experiments minute attention to the purity of the substances employed is indispensable: traces of foreign substances inappreciable to ordinary modes of analysis occasionally reveal themselves by their absorptive action on the chemical rays.

Among the various compounds submitted to examination, the fluorides are chemically the most transparent; then follow the chlorides of the metals of the alkalies and alkaline earths; the bromides are less diactinic, and the iodides show a striking diminution in this respect. The group most remarkable for its absorptive power is the nitrates. Nitric acid, whether simply dissolved in water, or combined with bases, has a specific power in arresting the chemical rays; the less refrangible portion it transmits freely, but intercepts the spectrum abruptly at the same point, whatever salt be employed, provided the base be diactinic. The chlorates, it may be remarked, are strongly diactinic.

Glass, even in very thin layers, absorbs the whole of the more refrangible rays.*

Diactinic bases, when united with diactinic acids, usually furnish diactinic salts: but such a result is not uniform: the silicates are none of them as transparent as silica itself in the form of rock crystal. Again, hydrogen is eminently diactinic, and iodine vapour, notwithstanding its deep violet colour, is also largely diactinic; but hydriodic acid gas is greatly inferior to either of them.

A hasty consideration of these experiments might lead to the conclusion that lenses of quartz, or of water enclosed in quartz, would be far superior to those of glass in ordinary use by the photographer. This, however, is not the fact. Glass is very transparent to the less refrangible portion of the chemical rays, extending beyond the violet end of the visible spectrum to a distance as much beyond the line H as the red end of the spectrum is below it; and these rays are precisely the most abundant and powerful chemical rays in the solar spectrum, which contains but few rays of a refrangibility much beyond this point, whereas in the electric arc these highly refrangible rays predominate.