by these rays in Nature little is understood. It has long been known that light exerts a powerful influence upon the organic world. Vegetation languishes in the absence of light, and flourishes when exposed to it. It was at first supposed that this power over plants resided in the chemical rays; but it is now known that the force that decomposes carbon dioxide in green leaves, and which is the foundation of the vegetative processes, is most active, not in the blue, but in the yellow space of the spectrum, where the actinic force is absent.

§ 2. Photographic Chemistry.

169. Chemical Reactions of Light. It was long supposed that the chemical rays act only upon a few substances, but the contrary is really the fact. So many substances are affected by it, and in so many different ways, that some think a ray of light cannot fall upon the surface of any solid without impressing upon it an enduring molecular change. Four kinds of effect may be here referred to: First, the elements, such as phosphorus, are altered by light in their allotropic forms (271). Second, light promotes chemical combination of the elements, as already shown (165). Third, it produces mechanical effects. If the beautiful ruby-colored crystals of arsenic disulphide are exposed to light for some months, they become pliant and fall to powder. Fourth, chemical compounds, as silver nitrate, are decomposed under the influence of light, and new compounds are formed.

170. Substances at the Basis of Photography. By exploring this subject, chemists have founded a new art of great importance, that of taking pictures quickly, cheaply, and accurately, by the direct action of light. Experi menters began to feel their way toward this result early in the present century; but the process only became successful in the hands of M. Daguerre, a Frenchman, who made it public in August, 1839; and since then it has undergone

the most rapid extension and. development. For photographic purposes the salts of silver are mainly used-silver iodide, bromide, and chloride, being the substances most generally employed. The unequal susceptibility of these compounds to the action of light gives great resources to the operator, and is the basis of this modern art.

171. Production of the Invisible Image.-Photographic pictures are taken, as is well known, by means of a cameraobscura, an instrument by which inverted images of ex

FIG. 92.

FIG. 91.

Photographic Camera.

B

ternal objects are produced in a dark

chamber in their natural colors. In Fig. 91 G represents a ground-glass slide upon which the image is formed, and which is viewed by the operator from behind. The glass plate is brought to the exact focus, first

by sliding the part M of the box in the part N, and then by turning the pinion V, which moves the lenses in the tube A B. Fig. 92 represents the lenses EL in the tube AB; two lenses having the effect of allowing a larger aperture, and increased light, with the same focal distance. A metallic or glass plate is then prepared in an obscurely lighted place, by coating it with the proper chemicals, and it is then said to be sensitive; that is, it is very susceptible to changes from the action of light. It is, therefore, kept protected from the daylight, until substituted for the glass slide G in the camera. The cap being then removed from be

Position of Lenses.

upon

fore the lenses, the light from the object to be taken falls. the sensitive surface. If silver iodide be used, such as Daguerre employed, twenty minutes will be necessary to get an impression. But if silver bromide, or chloride, be used, which are far more sensitive, the operation is quickened, and by mixing different chemicals any degree of sen sitiveness may be secured. If required, an impression may be obtained in the hundredth part of a second.

172. Developing the Picture. When the plate is removed from the camera, no effect is visible; the image has to be brought out or developed by a subsequent process. Daguerre effected this by exposing the plate to vapor of mercury, which, being condensed unequally upon the changed surface, evolved the lights and shadows, that became visible when the plate was washed with sodic hyposulphite, by which the unchanged silver iodide is dissolved away. At present the plates exposed in the camera are coated with collodion, in which the sensitive chemicals are contained. The pictures are developed by washing the surface with a solution of green vitriol, which, becoming mixed with the silver compounds, decomposes them, and precipitates the silver in the form of a fine black powder, that adheres to the exposed surfaces of the plate.

173. Negatives and Positives.-Pictures are now generally taken upon a transparent glass plate, in which the lights and shades are reversed, and these are called negatives. But from these negatives others are taken, and the effects are again reversed, which makes them true to nature-lights answering to lights, and shadows to shadows: these are called positives. They are copied, or printed from the negative, by placing sensitively-prepared paper surfaces against the negative, and exposing to sunlight. In this way, from a single negative, many positives may be obtained, while a little delicate retouching of the negative, with Indian-ink or a pencil, may remove defects, and improve all the positives printed from it. This is often de

sirable, as freckles and pimples upon the face are liable to be exaggerated in the photographic negative.

174. Varying Effect of Colored Lights.-Yellow and reddish light being chemically inoperative, the artist can carry on his manipulations by a dingy lamp-light or daylight passing through yellow glass; and as blue light is chemically most powerful, the reflected illumination of the sky is favorable for photographic effects. White clouds increase the chemical intensity of light, while gray clouds diminish it. Blue, indigo, and violet colors generally come out light in photographs, while yellow and red work dark. Hence dark-blue flowers on a light-yellow ground produce light flowers on a dark ground. Red, and also fair golden hair, becomes black, and yellow specks in the face produce black points in a picture. It is obvious from this unequal working of light that many-colored toilets must produce. discordant photographic results. "Persons of dark complexions, also stout persons, should prefer dark clothes; as it is well known that white clothing increases in appearance the fullness of the figure. Thin and pale persons are advised, on the contrary, to wear light clothes, as a pale complexion would appear even paler when contrasted with. black."-(Vogel.)

175. Celestial Photography.-The applications of photography in the arts are becoming constantly more valuable, and it is also an important resource of science in making quick and accurate representations, and in recording the workings of self-registering apparatus. Its astronomic indications are of especial interest. Enlarged photographs of the moon represent the details of its surface with surprising minuteness; and photographs of the stars are taken, which define their position with the greatest accuracy. In observing eclipses of the sun, this power of producing instantaneous pictures is invaluable; for the display in a total solar eclipse is grand, complex, and momentary-chromosphere, prominences, and corona, all burst

upon the view at once, and baffle every attempt at delineation. The corona is a vast, irregular luminous appendage surrounding the sun, reaching away to immense distances, only visible in eclipses, of unknown nature, and presenting the greatest diversity of aspects at different times. Photography is therefore eminently adapted to seize its peculiar and varying ap

pearances.

Fig. 93

FIG. 93.

N

Photograph of Total Solar Eclipse of 1868.

luminous line cutting the lower stratum of light was recorded upon the plate. Other photographs give a more even outline, and a wider circle of white light around the central body. The multiplication of such impressions in different places, and at different times, will be of the utmcst service in the investigations of solar physics.

CHAPTER VII.

SPECTRUM ANALYSIS.

176. Interest of the Subject.-The progress of science is full of surprises. A step is taken that seems so wonderful that nothing can surpass it; but it is soon eclipsed by something still more wonderful. With the remarkable