Photosphere

“ It is in the brilliant photosphere that the dark areas known as sun-spots appear. Some of these dark spots—they are dark only by contrast with the photosphere surrounding them—are of enormous size, covering many thousands of square miles of surface. What they are we cannot positively say. They look like great cavities in the sun's surface. Some think they are giant whirlpools. Certainly they seem to be great whirling streams of glowing gases with vapours above them and immense upward and downward currents within them. Round the edges of the sun-spots rise great tongues of flame. ”

“ Between photosphere and chromosphere is a very thin envelope, probably not over 700 miles in thickness, called the reversing layer. It is this relatively thin shell that is responsible for the absorption which produces the dark lines in the spectrum of the sun. Under normal conditions the filaments of the photosphere are radial, that is vertical on the [185] sun; but whenever eruptions take place, as during the occurrence of spots, the adjacent filaments are violently swept out of their normal vertical lines and these displaced columns then form what we view as the spot's penumbra. ”

“ The photosphere is that level in the atmosphere at which the general opacity cuts off the direct light from the interior; [355] in the case discussed the reversing layer is considered to be optically so thin that the general opacity is negligible. The selective opacity, depending on the natural absorption frequencies of the atoms present in the gas, gives rise [Pg 92] to the line absorption spectrum which we are about to consider; the region of sensible general opacity, represented by the photosphere, gives rise to a continuous spectrum corresponding to the continuous background in the star. ”

“ The photosphere is the visible layer of the Sun. When we talk about the temperature of a star, we generally refer to the photosphere’s temperature, and the light of the mentioned layer is also useful to do chemical analysis of the stars. ”

“ Hence, when a sufficient number of prisms is employed, the bright lines of the solar chromosphere may be seen in full sunshine, in spite of the greater intensity of the light emanating from the photosphere, which produces the continuous spectrum. ”

“ The interior is invisible. The spherical body which we popularly speak of as the sun is hidden from view by the opaque photosphere. This photospheric veil, including the sun spots; the brilliant faculæ and flocculi, projecting upward from the photosphere; the reversing layer, in effect immediately overlying the photosphere; the chromosphere, a stratum associated with and overlying the reversing layer; the prominences, apparently ejected from the chromosphere; and the corona, extending outward from the sun in all directions to enormous distances ”

“ Crossing the solar spectrum are hundreds and hundreds of dark lines. These could not at first be explained, because this fact of discriminative absorption was not known. We understand now. The sun's white light comes from the photosphere, but between us and the photosphere there is, as we have seen, another solar envelope of relatively cooler vapours--the reversing layer. Each constituent element in this outer envelope stops its own kind of light, that is, the kind of light made by incandescent atoms of the same element in the photosphere. ”

“ We see only bright upright lines. There is no continuous band or spectrum of prismatic colors. Now, just outside the solar photosphere, and between it and the chromosphere, is a layer of solar atmosphere which gives just such upright, bright lines. ”

“ The sun consists of a nucleus surrounded by a flaming atmosphere. The light of the nucleus would give us a continuous spectrum, like that of our common carbon-points; but having to pass through the photosphere, as our beam had to pass through the flame, those rays of the nucleus which the photosphere can itself emit are absorbed, and shaded spaces, corresponding to the particular rays absorbed, occur in the spectrum. ”

“ Of course, if the photosphere were really composed of glowing solid and liquid matter, or of masses of gas so compressed and so intensely heated as to give a continuous spectrum, no gas existing below the photosphere could send its light through, nor could its presence, therefore, be indicated in any spectroscopic manner. ”

“ Attempts, more or less plausible, have been made and are now pending to explain photosphere, spots and the reversing layer by means of the refraction of light within the sun's outer envelope of gases, and it seems altogether probable, in view of these combined activities, that a considerable addition to our store of knowledge concerning the sun may be expected in the not distant future. ”

“ The continuous background of the spectrum represents the photosphere—the deepest layers from which we receive light. The energy that produces it is practically the total energy output of the star. While the actual distribution of energy in the spectrum probably conforms, in general, to that of a black body, the observed distribution naturally deviates considerably. ”

“ The photosphere, as has been stated, is at an extremely small depth compared with the radius of the star. Taking the sun as an example, it is estimated by Russell and Stewart [93] that the reversing layer, which, with the chromosphere, is responsible for all the solar phenomena that can be spectroscopically studied, consists of about four tenths of a gram of matter per square centimeter of surface, and is only a few hundred kilometers in thickness. ”

“ Thus the action of our atmosphere, and, as is inferred from the observations, that of the solar atmosphere, is to absorb the short rays more than the long ones. The real color of the photosphere is blue ”

“ Only the photosphere may be to a greater or less extent solid or liquid. The dark lines that we see in the solar spectrum are produced by the absorption of a comparatively thin and cool layer of gas resting upon the photosphere. ”

“ On the surface of the photosphere, almost altogether within thirty degrees of the equator of the sun, a field corresponding approximately to the tropical belt of the earth, there appear from time to time the curious disturbances which are termed spots. These appear to be uprushes of matter in the gaseous state, the upward movement being upon the margins of the field and a downward motion taking place in the middle of the irregular opening, which is darkened in its central part, thus giving it, when seen by an ordinary telescope, the aspect of a black patch on the glowing surface. ”

“ The spots vary in size and number: sometimes but a single one appears, and as many as 80 have been visible at one time. Nor are they equally distributed over the solar surface. The region of the spots is generally confined to the part of the sun within about 30 of its equator, beyond which they are rarely seen.
The photosphere is the name given to the silver sea of light by which the spots are surrounded, and in which they seem to swim; it is the immediate source of illumination, and, viewed through a glass of low power, it appears flat and smooth, and of a uniform whiteness. ”

“ But you will say to me, it still remains true that the interior mass of the sun is less luminous than the photosphere; but since the superficial part constantly cools by radiation, it follows that there ought to be less heat, and, consequently, less brilliancy in the photosphere than in the interior mass. ”

“ When these spots are nearer the centre of the sun, we see more vertically into their depths, by which I frequently observe a third or cloud stratum, underlying the penumbra, and partially closing the opening, doubtless to screen the underlying globe (which, by contrast with the photosphere, is intensely black) from excessive light, or to render it more diffusive. ”

“ What is still more interesting, in a favorable state of the atmosphere, I can distinctly see over the whole surface of the sun, not occupied by large spots or by faculæ, a network of pores or minute spots in countless numbers, with dividing lines or faculæ-like depressions in the photosphere, separating each little hole, varying in size, some sufficiently large to exhibit irregularities of outline, doubtless frequently combining and forming larger spots. ”

“ In a favorable state of the atmosphere, and when spots are found lying more than halfway between the sun's centre and the margin, or better still, if nearer the margin, when the spots lie more edgeways to the eye, I can see distinctly the relative thickness of the photosphere and the underlying dusky penumbra, which lie on contiguous planes of about equal thickness, like the coatings of an onion. ”

“ Not only do we find that the chemical action is not extended over the whole length of the prismatic spectrum, but we discover that over spaces, which correspond with the maximum points of light and heat, a protective action is exerted. That is, that highly sensitive photographic agents, which blacken rapidly under exposure to diffused daylight, are entirely protected from change in full sunshine, if at the same time as a strong light is thrown upon them by reflection, the yellow and extra red rays are brought to bear upon their surface. ”

“ Let us imagine that the exposure has been made with an ordinary lens and camera, and that it is a landscape seeming to grow beneath the experimenter's eyes. At first only the most conspicuous objects make their appearance. But gradually the process extends, until finally every tiny detail is reproduced with marvellous fidelity to the original. The photographic plate, when developed in this way, is called a "negative." For in Nature luminous points, or sources of light, are bright, while the developing negative turns dark wherever light has acted. ”

“ The incident ray and the reflected ray form, together, what is called the passage of reflection, and this will therefore make the actual distance of an image to appear as far again from the eye as it really is. Any object which reflects light is called a radiant. The point behind a reflecting surface, from which they appear to diverge, is called the virtual focus.
Rays of light being reflected at the same angle at which they fall upon a mirror, two persons can stand in such a position that each can see the image of the other without seeing his own. ”

“ We thus see that light is not a simple and uniform radiation from the sun, but is made up of a large number of separate rays, each of which produces in our eyes the sensation of a distinct colour. Light is now explained as being due to vibrations of ether, that mysterious substance which not only permeates all matter, but which fills space at least as far as the remotest of the visible stars and nebulæ. The exceedingly minute waves or vibrations of the ether produce all the phenomena of heat, light, and colour, as well as those chemical actions to which photography owes its wonderful powers. ”

“ Let D E, Fig. 250, a beam of the sun's light, pass through a small opening in a shutter into a dark room. The rays will pursue a straight course, and if a screen be placed at F they will make a spot of white light. But if a glass prism, A B C, be held in the position represented the rays will be refracted, and when received upon the screen M N the light will be separated into seven colors in the order which is given. The figure thus produced is called the solar spectrum. Observe why it is that the colors are separated. It is because they are refracted unequally. ”

“ The waves of light produce in sensitive substances chemical changes; new compounds are formed; why should the long waves of red light produce compounds that are especially capable of reflecting long waves, or red light? When we undertake to make a photograph in color, in effect we ask one and the same chemical substance to reflect for us long, medium, or short waves, red, green, or blue light, according as it has been acted on by waves of greater or lesser length. The demand seems to me preposterous.
The hope for photography in color lies in a different and less independent direction. ”

“ As we look at the image on the ground glass, it will be evident that as we stop down our lens, the more remote objects are gradually brought into view with a sharp outline, we shall discover that the image on the ground glass becomes less and less distinct, which shows very clearly that we are admitting less light, and the lesson to be learned is that when we make the exposure we must give a corresponding increase in time as the amount of light admitted decreases. ”

“ But more than this, these sky spots in a dark background are air holes. They enable us to breathe in the picture, giving a decided sense of atmosphere. When well subordinated they offer no distraction to the subject, but give to the picture a depth. When no other object is introduced, a gradation is serviceable. Much may be thus suggested and besides the depth and air properties thus introduced, such variety of surface excites visual motion. The eye always follows the course of light from the shadow. ”

“ This is due to no other cause than that which produces [Pg 112] the rainbow and the spectrum—the separation, or "dispersion" in their passage through a refracting medium, of the variously coloured rays composing a beam of white light. In an ordinary lens there is no common point of concentration; each colour has its own separate focus; and the resulting image, formed by the superposition of as many images as there are hues in the spectrum, is indefinitely terminated with a tinted border, eminently baffling to exactness of observation. ”

“ These same rays may be collected and their intensity again increased.
Rays of light are reflected from one surface to another; Refracted, or bent, as they pass from the surface of one transparent medium to another; and Inflected, or turned from their course, by the attraction of opaque bodies. From the first we derive the principles on which mirrors are constructed; to the second we are indebted for the power of the lenses, and the blessings of sight,--for the light acts upon the retina of the eye in the same manner as on the lens of a camera. ”