Lens

“ The lens is so made that the front part of it can bend just as the distance changes. You have only to look at an object and the lens is adjusted without the slightest effort or knowledge on your part.
You may wonder how light waves can pass through a substance as solid and as hard as glass or through the eye. You will remember I told you in the beginning of this chapter how ether got into every little space, even in metals and glass, as well as that it filled all the great space between the stars. ”

“ A lens is a transparent substance of a different density from the surrounding medium, and terminating in two surfaces, either both spherical, or one 64 spherical and the other plain. It is usually made of glass, but may also be formed of any other transparent substance, as ice, crystal, diamond, pebbles, or by fluids of different densities and refractive powers, enclosed between concave glasses. Lenses are ground into various forms, according to the purpose they are intended to serve. ”

“ Lenses are pieces of glass having curved surfaces. When a beam of light passes through a lens it is also bent out of its original direction, or refracted.
A convex lens, see Fig. 146, is a lens which is thicker in the middle than it is at the edges. A convex lens is used for magnifying an object; or for forming an image so that it can be magnified by another lens as in a telescope, for forming an image on the screen of a camera, and for bringing the heat waves of the Sun to a focus, as with a burning glass. ”

“ If a lens is now held an inch, or more, according to its focal length, over the part of the line A B, and the slightest amount out of its centre, that part of the line A B which passes under the lens will be seen in the direction of the figures 1.2; but if the lens be now moved a little out of its central position in the opposite direction, the line B C will be observed at 3.4, or below B C. A lens is a magnifying glass because it dilates or spreads out from its centre the objects observed through it Therefore whatever is magnified by it is seen a little out of its axis or centre. ”

“ The normal focus is for distant objects. When we read a book the eye adapts itself to the work. The ligaments relax and the lens decreases in diameter while thickening at the centre, until its curvature is such as to focus all rays from the book sharply on the retina. If we suddenly look through the window at something outside, the ligaments pull on the lens envelope and flatten the curves.
This wonderful lens is achromatic, and free from spherical aberration and distortion of image. ”

“ The lens is just behind the pupil or central aperture in the iris or coloured ring at the front of the eye, and behind the cornea which is to the eye what a watch-glass is to a watch. If the lens of the eye be removed from a newt, as it is from human beings in the operation for cataract, the animal will grow another one. ”

“ A lens can be defined as a transparent medium which, owing to the curvature of its surfaces, is capable of converging or diverging the rays of light passed through it. According to its curvature it is either spherical, cylindrical, elliptical, or parabolic. The lenses used in optics are always exclusively spherical, the glass used in their construction being either crown glass, which is free from lead, or flint glass, which contains lead and is more refractive than crown glass. ”

“ A lens acts by refraction and brings rays to a focus. If light be simple it acts well, but if ordinary white light fall upon a lens, its different constituents have different foci; every bright object is fringed with colour, and nothing like a clear image can be obtained. ”

“ It is the lens that forms the pictures in your eye, on a photographic plate or film, and on a moving-picture screen. And a lens is usually just a piece of glass or something glassy, rounded out in such a way as to make all the spreading light that reaches it from one point come together in another point, as shown in Figure 69.
As you know, when light goes out from anything, as from a candle flame or an incandescent lamp, or from the sun, it goes in all directions. ”

“ For these impressions we must rely solely upon the lenses which God has given us; and as a painter I congratulate myself daily that the lens of the human eye was designed not at all after the pattern of the lenses adapted to the camera, the microscope, and the various other scientific instruments. As we are now provided, nature is infinitely beautiful to us ”

“ The lens causes to be formed on the back of the eye a clear picture of whatever you are looking at. When you focus a camera, you move the back towards or away from the lens. When you focus your eye you cannot move the back of the eye, but you can make the lens more or less convex as may be needed to make a clear picture. ”

“ A lens is glass ground in such a form as to collect or disperse the rays of light which pass through it. These are of different shapes; and they have, therefore, received different appellations. A plano-convex lens has one side flat, and the other convex; a plano-concave lens is flat on one side, and concave on the other; a double convex lens is convex on both sides; a double concave lens is concave on both sides; a meniscus is convex on one side, and concave on the other. ”

“ Now, in the same manner, a lens throws a picture of objects to which it is directed on the retina of the eye, and when that picture is larger than the image made in the eye by the object, without the aid of the lens, it is magnified, or the lens has served as a microscope, so called, from its use in seeing small objects, from mikros, small, and skopeo, to see. ”

“ We may also use a much stronger lens, and so get the object very near the lens and eye, as in Fig. 35, where 70a b is the object so near the eye that, if it were not for the lens L, its image would not come to a focus on the retina at all. The effect of the lens is to make the rays proceeding in a cone from a and b less divergent, so that after passing through it, they proceed to the eye-lens as if they were coming from the points A and B, a foot or so away from the eye, and so the object a b appears to be a much larger object at a greater distance from the eye. ”

“ If our lens is one of large aperture, that is to say admits a considerable amount of light and is also what is known as a long focus lens we shall find, when we view the scene on the ground glass, that when the tree is sharply focussed, the cottage and hill are not clear. When we rack in the camera to get the cottage sharply focussed, the tree and hill will be un-sharp. Similarly when we focus on the hill the tree and cottage remain out of focus. ”

“ The magnifying power of a lens depends upon its 'focus' multiplied by its distance from the screen, and the focus in the case of a simple lens is easily determined by the familiar 'burning-glass' experiment, that is by focussing an image of the sun upon a piece of paper and measuring accurately {67}the distance the lens must be away to produce the most concentrated spot. ”

“ When we look through a mirror-telescope (or, let us say, when we photograph with it, for that makes the explanation simpler) , then we see that the objects in the middle of the picture are in good focus, but not those at the edges. Perhaps many of you have got cameras of your own, kodaks or brownies, or some other sort, and you may know the difference between good and bad lenses—with a good lens all the picture is in focus, with a poor one the edges show up fuzzy. ”

“ Working at F 4.5, the lens gives you the opportunity of getting the broadest effects in landscapes or the softest in portraits. As a rule these are not pleasing to most people when enlarged. I therefore usually work with the lens at F 6 or F 8, which gives a delightful image with distinct contours and a definite softness to the outlines, making beautiful enlargements which are sharp enough for bromoil or gum. ”

“ Different Positions of Object and Image.—The principal points are always near the surfaces limiting the lens, and consequently the lens divides the direct pencil containing the axis into two parts. The object can be either in front of or behind the lens as in fig. 10. If the object point be in front of the lens, and if it be realized by rays passing from it, it is called real. If, on the other hand, the object be behind the lens, it is called virtual; it does not actually exist, and can only be realized as an image. ”

“ If now the field lens of the ocular be made of heavy flint glass and the separation of the lenses suitably adjusted, the stronger refraction of the field lens for the blue pulls up the blue focus and brings its image to substantially the dimensions of the red, so that the eye lens performs as if there were no overcorrection of the objective. ”

“ As the lens can only deflect their path to a fixed degree, they will not, after passing the lens, come together until they have reached a point, F1, further from the lens than F. The nearer we approach O to the lens, the further away on the other side is the focal point, until a distance equal to that of F from the lens is reached, when the rays emerge from the glass in a parallel pencil. ”

“ You know that the lens, like the air, is of varying thicknesses or densities, and you know that in the ordinary camera the rays of light, passing through the upper part of the lens, are refracted or bent toward the perpendicular so that they reach the ground-glass “finder” at the bottom ”

“ B. The same effect in concentrating the rays, is produced by the refraction with a lens, as by the reflection from a concave mirror: in the first, the rays pass through the glass and converge to a focus, behind it, in the latter, they are reflected from the mirror, and brought to a focus, before it. A lens, when used for the purpose of collecting the sun's rays, is called a burning glass. I have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus ”

“ The numerical aperture of a lens determines all its essential qualities; the brightness of the image increases with a given magnification and other things being equal, as the square of the aperture; the resolving and defining powers are directly related to it, the focal depth of differentiation of depths varies inversely as the aperture, and so forth. ”

“ If the light be placed at the principal focus of the lens, the rays leaving the lens will be parallel. If the light be brought nearer the lens, the rays leaving it will spread out as shown in Figure 87; they will leave the lens in a direction such as to make upon an observer from in front the impression that they are coming from the point C behind the lens. ”

“ The natural form of the lens is more convex than is consistent with distinct vision, except for very near objects. The tension of the suspensory ligament, which is attached to the front of the lens all round its edge, renders the anterior surface of the lens much less curved than it would naturally be. ”

“ Let A (fig. 51.) represent the lens placed on a pedestal; the rays of light passing through this lens from distant objects will converge towards a focus at F. If a person then, place his eye at E, a certain distance within the focal point, he will see distant objects magnified nearly in the proportion of the focal distance of the lens to that of the eye; and when the lens is very broad—such as the 26 feet lens mentioned above—two or three persons may look through it at once, though they will not all see the same object. ”

“ By moving the eye about its centre of rotation M the whole field can be examined. The margin of the mount of the lens serves as the diaphragm of the field of view. The selection of the rays emerging from the lens and actually employed in forming the image is undertaken by the pupil of the eye which, in this case, is consequently the exit pupil of the instrument. ”

“ The lens is not always a perfect optical instrument, on the contrary, it is as a rule somewhat defective. Of course, a great many details concerning the process of lens regenera­tion have still to be worked out. ”

“ Lens.—A lens is made of glass and prisms graded in strength, one surface curved, and has the power of refracting or changing the direction of the rays of light. A prism is wedge-shaped and bends rays of light towards its base. A great many people are troubled with their eyes, much more than years ago. ”

“ The deduction of the cardinal points of a spherical glass lens in air from the relations already proved is readily effected if we regard the lens as a combination of two systems each having one refracting surface, the light passing in the first system from air to glass, and in the second from glass to air. ”

“ The focal length of a lens is determined by the convexity of its surfaces and the refractive power of the material of which it is composed, being shortened either by an increase of refractive power, or diminution of the radii of curvature of the faces of the lens. ”

“ In considering the action of a lens we can regard any lens as being built up of a number of prisms with curved faces in contact. Such a lens is shown in Fig. 67, the light rays being refracted towards the base of the prisms or towards the normal, as already explained; while the top half of the lens will refract all the light downwards, the bottom half will act as a series of inverted prisms and refract all the light upwards. ”

“ The flat faces of the two lenses are turned into contrary directions in this eye-piece—one facing the object-glass, and the other the eye of the observer; and as the image formed at the focus of the object-glass, lies parallel to the flat face of the contiguous lens, every part of the field of view is distinct at the same adjustment, or, as opticians say, there is a flat field, which, without a diaphragm, prevents distortion of the object. ”