Visible light

“ Rays of light emitted or reflected from a body entering the pupil of the eye, are so refracted by the humours of it, as to be united at the surface of the retina, and so make images of the objects, by means of which they are visible to us ”

“ The light of this orb is subject to three principal laws requiring notice, viz., transmission, colour, and refraction by transparent media.
1st. The sun or any body in a state of combustion contains independent light; and striking opaque or non-luminous bodies in straight lines or rays, which are intercepted and reflected at the same angle as that of incidence. When these same rays strike the eye of man, the illuminated body becomes visible. ”

“ That colours, as visible to the eye, are all formed in the solar light, and their appearance depends upon the absorption of some of the rays of light, and the reflection of others, is a doctrine which followed the discoveries of Sir Isaac Newton. ”

“ Light naturally travels in a straight line. It is deflected only when it passes from one transparent medium into another—for example, from air to water—and the mediums are of different densities. We may regard the surface of a visible object as made up of countless points, from each of which a diverging pencil of rays is sent off through the ether. ”

“ The brighter the objects reflected, the larger the angle at which reflection is visible; it is always to be remembered that, strictly speaking, only light objects are reflected, and § 7. The inherent hue of water modifies dark reflections, and does not affect bright ones.that the darker ones are seen only in proportion to the number of rays of light that they can send; so that a dark object comparatively loses its power to affect the surface of water, and the water in the space of a dark reflection is seen partially with the image of the object, and partially transparent. ”

“ The same effect may be observed when looking at the setting sun, or a flaring gas light. If the light at which we look is coloured, we shall see the complementary colour in the impression left on the retina. ”

“ Light, through the medium of the eye, not only unfolds to us the persons of others, in all their minute modifications and peculiarities, but exhibits us to ourselves. It presents to our own vision a faithful portrait of our peculiar features behind reflecting substances, without which property we should remain entirely ignorant of those traits of countenance which characterize us in the eyes of others.
But, what is the nature of this substance we call light, which thus unfolds to us the scenes of creation? ”

“ To a short-sighted person, colours run together and intermix, outlines disappear, blues and reds and yellows become russets or browns, the lamps or candles of an illumination spread into an unmeaning glare, or dissolve into a milky way. He takes up an eye-glass, and the mist clears up; every image stands out distinct, and the rays of light fall back upon their centres. ”

“ Wherever I move, whether by rays or waves of light, from the farthest star to the nearest leaf with its complexity of vein and tint, there comes to that pinpoint of space, the eye, a multitudinous vision. ”

“ And indeed it is the light itself, which after a certain manner, either mixed with shades or other-wise, strikes our eyes and immediately produces that motion in the organ which gives us the color of an object. ”

“ If, for example, we look steadfastly at the sun for a second or two, and then immediately close our eyes, the image, or spectrum, of the sun remains for a long time present to the mind, as if the light were still acting on the retina. ”

“ Whenever we have the perception of light, we habitually have, along with it, the perception of objects; that is, of all sorts of colours, all sorts of shapes, all sorts of magnitudes, all sorts of distances, and so on. With the idea of light, then, are indissolubly associated the ideas of all sorts of objects; of extension in all its modifications, colour in all its modifications, motion in all its modifications; the word light, therefore, serves as a name, not merely of the fluid which acts upon the eye, but of that along with its innumerable associations. ”

“ The space through which light passes is, by opticians, called a medium, and it is observed, that, when light passes through a medium, either absolutely void, or containing matter of an uniform density, and of the same kind, it always proceeds in straight lines.
3. Those rays of light which come directly from a luminous body to the eye, only give us a perception of light; but when they fall upon other bodies, and are from them reflected to the eye, they give us an idea or perception of those bodies. ”

“ Rays of light, which are in themselves invisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannot hear a person speak—that we only hear the vibrations of the air. ”

“ I wish thee to believe that light in the bodies that we call luminous is nothing more than a very brisk and violent motion, which, by means of the air and other transparent media, is conveyed to the eye, exactly as the shock through the walking-stick reaches the hand of a blind man. ”

“ In one word, the sphere of darkness in which I floated seemed to be filled with points of light, while the absolute blackness that surrounded them, the absence of the slightest radiation, or illumination of space at large, was strange beyond expression to an eye accustomed to that diffusion of light which is produced by the atmosphere. ”

“ What we shall see with it is far from being that sphere of dazzling light, everywhere equally brilliant, which we have been accustomed to consider the sun. The eye ranges over a vast surface, presenting at one view over five thousand times the entire area of our globe, to find everywhere diversity of shade. ”

“ Uses of Refraction. If it were not for refraction, or the deviation of light in its passage from medium to medium, the wonders and beauties of the magic lantern and the camera would be unknown to us; sun, moon, and stars could not be made to yield up their distant secrets to us in photographs; the comfort and help of spectacles would be lacking, spectacles which have helped unfold to many the rare beauties of nature, such as a clear view of clouds and sunset, of humming bee and flying bird. ”

“ Nor can any brilliant light, a crisp, snapping [106] light—a glint of the sun's rays, for instance, on the break of the surf, or on the round of a glossy leaf, reflecting like a mirror the opaque sky—ever be achieved by careful working around the edges of an unwashed speck of paper—the transparent man's only means of expressing a high light. ”

“ The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a polarised condition of light which imparts to vision the sensation of a delicate blue. ”

“ In the original picture these contrasts would depend partly on the color of the various objects; but here, in the black and white reproduction, we may think of the pattern simply as one of very dark spots and very light ones, threaded together by others of varying depths of greyness. Again, what an important part the sky plays! It is a flood of light, against which everything forms a silhouette, [4] more or less dark, relieved by spots and streaks of light. The water, but for the pathway of reflection, is shrouded in shadow. ”

“ The phenomena discernible by the sense of sight are explained, but only by bringing into the range of a purer and more piercing vision phenomena infinitely more wonderful. If one could see through some amazing extension of visual power, or if even the imagination could adequately picture, the rush of light waves of all orders of length upon the line of crystal breakers, their deflection in all directions, their separation into their various orders of wave-length ”

“ For instance if the sun would have shone always without setting, we could have formed no idea of light, knowing simply that objects have certain colours. But the setting of the sun revealed to us the nature of light by comparing it with darkness. ”

“ To do this the student must find the lightest light and darkest dark in the subject before him and, having found it, adhere to it to the end of his work. For as the sun dominates the sky and earth so do its rays dominate parts of the whole, making more luminous than the rest only one object upon which its light falls. ”

“ We may think of the air around us as a vast ocean, through which waves conveying light and sound are constantly travelling. When a sound-wave strikes the ear, we hear; when a light-wave, moving like a water-wave, reaches the eye, we see. Light comes chiefly from the sun: it is beautiful to think, is it not?—of waves of light streaming always, day and night, from that wonderful sun so far away, and coming, wave after wave, to paint beautiful pictures on our eyes! ”

“ Immediately over the pupil of the eye we find the faculty of Vision or sense of Sight, marked Light, which runs into a sense of Shade at the inner angle of the eye, by which two perceptions everything in nature except colors is recognized. Light extends up into Color. ”

“ But before going into all the phases of Lights and Optics we will proceed to explain the structure of the eye, as it is through that organ that we are enabled to appreciate light and its marvellous effects. ”

“ Light, which embellishes with so much magnificence a pure and serene sky, by means of innumerable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. ”

“ Let us first try and understand the conditions under which phenomena are presented to us. In our perception of sight, we find the greater the light, the greater the shadow; a light placed over a table throws a shadow on the floor, though not sufficient to prevent our seeing the pattern of the carpet; increase the light and the shadow appears now so dark that no pattern or carpet can be seen; not that there is now less light under the table but the light above has to our sense of sight created or made manifest a greater darkness. ”

“ At the outset we have such purely physical questions as the nature of light, the cause of its emission, the mode of its propagation, the difference in the waves which give rise to the various color sensations, the principles of absorption, of reflection and of refraction, and the nature of material surfaces whereby they acquire their characteristic colors. ”

“ A corresponding effect occurs when one suddenly exposes to darkness the eye adapted to light. One can easily observe what goes on then, if, after looking for a time at an undifferentiated light surface such as the evenly luminous sky, one covers the opened eyes with the hollowed hands. ”

“ Probably the small amount of light reflected by transparent glass, which gives a passage to the greater part of the rays, may serve to illustrate the small amount of light reflected from black surfaces, which absorbs the greater portion of light. ”

“ All these beauties are indeed derived from the imponderable and invisible agent, light; and the variety and changefulness of the effects we may constantly observe show that light possesses the power of impressing our visual organs in a thousand different ways, modified by the surrounding circumstances, as witness that ever-shifting transformation scene—the sky. ”

“ Material bodies which to the human eye are opaque, may to them be as transparent as crystal or the atmosphere to man. The degree of light necessary to their vision of objects may be as nothing compared with that required by the human eye ”

“ This invisible electrical energy, without brush or color, paints the gorgeous beauties of the rainbow, and photographs in every ray of light and on every human eye the moving panorama of every passing scene. ”

“ The trained eye is capable of making a comparison between two surfaces illuminated by different sources of light, and pronouncing upon their equality or otherwise in respect of brightness, apart from a certain colour difference ”

“ In the case of light, we are in the presence of rays that are not luminous in sunlight, or at least are not seen by us, which are energetically absorbed by most transparent media and especially by the humors of the eye, for which we give ourselves no concern whatever in current life, and which nevertheless act upon the retina. ”

“ Self-illuminating and dark bodies, transparent and opaque, straight path of light, shade, rate of transmission, reflection, refraction, diffusion of color, convex and concave lenses, microscope, telescope, camera, the eye, spectacles. ”

“ And, while it is the light that enables us to see the object, it is the degree of light on some parts of it and the various degrees of darkness on others that enable us to realize the shape of it. In other words, the contrast of light and dark, received by the eyes, communicates to our brain the sense of form and bulk. ”

“ Light was regarded as of the nature of a stream of particles emitted from the object seen, and the act of vision was considered as a collision of this emanation with an emission of something from the eye itself, called in mediaeval writings the ‘visual spirit’. ”

“ Light and its phenomena—Magic pictures—The optical paradox—Chinese metallic mirrors—Effect of an optical instrument on a superstitious mind—Origin of photography—The Talbotype—The Daguerreotype—Sunlight pictures.
The cause of those sensations which we refer to the eyes, or that which produces the sense of seeing, is light. The phenomena of vision have always been regarded as among the most interesting branches of natural science. The knowledge of the laws which regulate the phenomena of light, constitutes the science of optics, which explains the cause of many most striking illusions. ”