Chapter D: D, an apparatus for holding a small tube, that the spectrum given by (10)

The expediency of the regulation appointing three men to be always at the lighthouse may be illustrated by an incident which occurred about the beginning of the present century at the lighthouse on the “Smalls,” a rock in the Bristol Channel. Two keepers held watch over the light on that rock, which for months together is sometimes cut off from all communication with the shore. At the time alluded to, after the weather had for two weeks prevented access to the lighthouse, it was rumoured among the seafaring men of the neighbouring ports that something was wrong at the “Smalls,” for a signal of distress had been observed; but the boats could not go within speaking distance, although many attempts were made to reach the rock. The relatives of the men became anxious, and night after night watched for the light. But the light never failed to appear at the proper hour. After four months came calmer weather, and then a boat brought to shore one lightkeeper alive, the other dead. What the former felt when he found his comrade to be dying in their dreadful isolation, or what his emotions were when he found himself there alone with the lifeless body, is not recorded. But the thought occurred to him that he must not commit the body to the waves, lest any suspicion of foul play might fall upon himself. He therefore contrived a sort of coffin for the dead man, and dragging it up to the gallery of the lighthouse, tied it there. Punctually and faithfully for four long months did he perform all the duties of his position, keeping watch from twilight till dawn in that lonely light-room, while his ghastly charge remained there within sight. But he came on shore strangely altered—a sad, silent, gloomy, worn man—so that even his intimate friends hardly knew him.

Here we close this brief account of the modern lighthouse, and of its beautiful appliances, by which Science “has given new securities to the mariner,” in addition to those with which she furnished him when she showed him the use of the compass, supplied him with the chronometer, and placed the sextant in his hands. How anxiously must the seaman who has been prevented by unfavourable skies from ascertaining his exact position, and has been trusting to the log and the compass to work his reckoning, scan the horizon for the first glimpse of the hospitable light beacon, which seems to say that the country he is approaching has been watching for his coming, and welcomes him to its shores.

FIG. 307.
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PHOTOGRAPHY.

No other of our nineteenth century inventions is at once so beautiful, so precious, so popular, so appreciated as photography. It is exercising a beneficial influence over the social sentiments, the arts, the sciences of the whole world—an influence not the less real because it is wide-spread and unobtrusive. The new art cherishes domestic and friendly feelings by its ever-present transcripts of the familiar faces, keeping fresh the memory of the distant and the dead; it keeps alive our admiration of the great and the good by presenting us with the lineaments of the heroes, the saints, the sages of all lands. It gratifies, by faithful portrayals of scenes of grandeur and beauty, the eyes of him who has neither wealth nor leisure for travel. It has improved pictorial art by sending the painter to the truths of nature; it has reproduced his works with marvellous fidelity; it has set before the multitude the finest works of the sculptor. It is lending invaluable aid to almost every science. The astronomer now derives his mathematical data from the photograph; by its aid the architect superintends the erection of distant buildings, the engineer watches over the progress of his designs in remote lands, the medical man amasses records of morbid anatomy, the geologist studies the anatomy of the earth, the ethnologist obtains faithful transcripts of the features of every race. To the mind of an intelligent reader numberless instances will present themselves, not only of the utility of photography in the narrower sense of the term, but of its higher utility in ministering to our love of the beautiful in art and in nature.

Effects produced by chemical changes to which the rays of the sun give rise are matters of common observation. The fading of the colour in the portions of a fabric which are exposed to the light is a familiar instance; and the bleaching of linen under the influence of sunshine in the presence of moisture is a well-known operation. Decompositions produced by light in certain compounds of silver soon attracted the attention of chemists, and the remarkable activity of the solar rays in causing the combination of hydrogen and chlorine gases has been even made the means of measuring the intensity of light. When equal volumes of these two gases are mixed together in the dark, they may be kept for an indefinite period without change, provided only that the mixture be preserved from access of light. But the instant it is exposed to the direct rays of the sun, or to an intense light, such as that of burning magnesium, the two gases suddenly unite with a loud explosion, in which the glass vessel containing them is shattered into atoms. The product is an intensely acid invisible gas, called hydrochloric acid; and if the mixture is exposed to the diffused light of day, instead of the direct rays of the sun, then the production of hydrochloric acid will take place gradually, and with a rapidity depending on the intensity of the light.

Of vastly more importance than the small operations of the laboratory and the bleach-field are the changes which the sun’s rays silently and unobtrusively effect in the vegetable world. The chemical effect of light here appears to reside in its power of separating oxygen from substances with which it is combined. The green parts of plants absorb from the atmosphere the carbonic acid gas, which is constantly produced by the respiration of men and animals, and by combustion, and other processes. Under the influence of sunshine, this carbonic acid is decomposed within the tissues of the plant; the oxygen is restored to the atmosphere; the carbon with which it was united is retained to build up the structure of the plant. In a similar manner light separates the oxygen from the hydrogen of water, and the former gas is given off by the leaves, while the hydrogen enters into the composition of the plant. The carbon, which forms so large an element in the food of plants, is chiefly obtained in this way; and the abundance of the supply of oxygen thus thrown into the atmosphere may be inferred from the fact that a single leaf of the water-lily will in the course of one summer give off nearly eleven cubic feet of oxygen. But for this continual restoration of oxygen to the atmosphere, animal life would soon disappear from the face of the earth. It is the office of the vegetable world not only to furnish a supply of organic matter as food for animals, but when the materials of that food have been converted into oxidized products in the animal system, and returned to the atmosphere as carbonic acid and aqueous vapour, the sunshine, acting on the vegetable structure (chiefly on the delicate tissue of the leaf), tears apart the oxygen and the other substance. These are, therefore, once more capable of combination, by which they may again supply the animal with heat and the other energies of life.

Those actions of light which have been last referred to are called by the chemist _reducing actions_, a term which he applies to the cases in which a compound is made to part with its oxygen or other similar element: when the remaining ingredient is a metal, the operation by which the other has been removed is always called _reduction_. On the other hand, the inverse operations by which oxygen, chlorine, &c., are fixed upon other bodies, are distinguished as processes of _oxidation_. Light is the means of determining each of these kinds of changes, according to the conditions and the nature of the substances exposed to its action. Thus moist chloride of silver will retain its white colour if preserved in the dark; but if exposed to sunlight, it quickly acquires a violet tint, which deepens in intensity until it has become black. The dark matter was formerly admitted to be silver; for it was known that the finely divided metal has this appearance, that during the process the compound gives off chlorine, and that when nitric acid is poured upon the darkened matter, reddish fumes are given off, exactly as when the acid acts upon pure silver. The use of silver nitrate as a marking-ink for linen depends upon a similar alteration of the salt within the fibres; and the same reduction takes place when to a solution of the nitrate in water organic matter is added. If a piece of white silk be dipped into a solution of chloride of gold, and exposed to the sun’s rays while still wet, the silk becomes first green, then purple, and finally a film of metallic gold will be found overspreading its surface. Many other chlorides and analogous compounds are similarly affected by sunlight. On the other hand, chlorides, as we have already seen, and oxygen, fix on hydrogen and on organic substances with greater energy under the influence of light. A large series of chemical compounds are obtained by means of the augmented affinity of chlorine for hydrogen induced by the rays of the sun.

It was in availing himself of an action of the latter class that, in 1813, Joseph Nicéphore Niepce[11] established photography; for he was the first to obtain a permanent sun-picture. Twelve years before this, Wedgwood and Davy had copied paintings made on glass, and the profiles of objects, the shadows of which were projected upon a piece of white paper, or white leather, saturated with a solution of nitrate of silver. The images so obtained could not be fixed, as no means was then known of removing the silver salts which had not been acted upon during the exposure; and the pictures soon blackened in every part when exposed to the light. The application of the _camera obscura_, and the fixing of the image so obtained, define the commencement of the art of photography. The process of Niepce, which was termed _heliography_, was conducted by smearing a highly polished metallic plate with a certain resinous substance known as “bitumen of Judæa,” and this was exposed to the image formed in the camera for some hours. The action of the light was such, that the resin, which before exposure was soluble in oil of lavender, became insoluble in that substance. Hence, on treating the plate after exposure with that solvent, only the deep shadows dissolved away, the lights being represented by the undissolved resin. The brightly polished parts of the plate, which were uncovered by the removal of the resin, appeared dark when made to reflect dark objects, while the resin remaining unchanged on the plate appeared light in comparison.

Footnote 11:

Born at Chalon-sur-Saône, died 1833.

In 1826 a French artist, named Daguerre,[12] who had already made some reputation as a painter of dioramas, entered into a sort of partnership with Niepce, into whose process he introduced some improvements; but, dissatisfied with the slowness of this proceeding, he invented a process of his own, by which pictures of great beauty could be produced with all the shadows, lights, and half-tints faithfully rendered; while the time of exposure in the camera was reduced to twenty minutes. In this process the burnished surface of silver formed the shadows. A plate of copper, coated with pure silver, had the silvered surface polished to the highest degree, and it was then exposed to the vapour of iodine until a thin yellow film had been produced uniformly over the silver. It was then placed in the camera; and, although when withdrawn no image was perceptible, a latent image was nevertheless present; for when the plate was exposed to the vapour of mercury, that substance attached itself to the parts of the plate in proportion as they had been acted upon by the light. Means were adopted by Daguerre for fixing the picture; and after his processes had been made public in 1839, several important improvements were proposed by other persons. By using bromine as well as iodine the sensitiveness of the plates was so much increased that the time required for exposure was reduced to two minutes, so that about the year 1841 portraits began to be taken by this process.

Footnote 12: