Lens (eye)

“ The magnifying power of a single lens depends upon its focal length, the object being in fact placed nearly in its principal focus, or so that the light which diverges from each point may, after refraction by the lens, proceed in parallel lines to the eye, or as nearly so as is requisite for distinct vision. In Fig. 3, A B is a double convex lens, near which is a small arrow to represent the object under examination, and the cones drawn from its extremities are portions of the rays of light diverging from those points and falling upon the lens. ”

“ 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. ”

“ The total deflexion of the light, to produce the magnifying power, is equally divided between the two glasses,—the most favourable condition for diminishing that distortion which is always perceived in looking obliquely through a lens; and the field of view is greatly enlarged in proportion to the size of the eye-lens, being such as would require, to produce the same magnifying power, a single lens of the much greater semi-diameter bd, found by drawing Qb parallel to qB and erecting bd. ”

“ If the real image produced by the objective coincides with the collective lens, only the inclination of the principal rays is altered, the form of the cone being affected only to a very small extent. The lens nearer the eye, which has about the same focal length as the collective lens, is distant from it by about its focal length. The eye-lens converts diverging pencils into parallels. Both lenses together form the exit-pupil of the objective behind the eye-lens, so that this image, the exit-pupil of the total system or the Ramsden circle, is accessible to the eye of the observer. ”

“ The single lens has a very small sharp field, hardly 10° in angular extent, the image falling off rapidly in quality as it departs from the axis. If plano-convex, as is the eye lens of common two-lens oculars, it works best with the curved side to the eye, i.e., reversed from its usual position, the spherical aberration being much less in this position. ”

“ These anastigmatic lenses, which are manufactured up to × 40, are chromatically and spherically corrected, and for a middle diaphragm the errors of lateral pencils, distortion, astigmatism and coma are eliminated. “Peephole” observation is employed, observation being made by moving the head and eye while the lens is held steady. Even in powerful magnifications a good image exists in all parts of a relatively large field, and the free working distance is fairly large. ”

“ 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. ”

“ 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. ”

“ Now, the human eye has to fulfil the requirement of projecting clearly-defined images on the retina, whether they are but a few inches off, or at an immeasurable distance. The eye being strictly subject to lenticular laws, either the space between the lens and the retina must have the power of varying, or the lens itself, by a change of form, must exercise now a stronger now a weaker power of refraction. ”

“ 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. ”

“ To accommodate the eye to different distances the lens is capable of altering itself with great precision and rapidity. When we look at a near object, the anterior surface of the lens bulges forward, becoming more convex the nearer the object; the more distant the object the more the lens is flattened. When the transparency of the cornea, the crystalline lens, or any of the contents of the globe of the eye is destroyed, either partially or entirely, then will partial or total blindness follow, since no image can be formed upon the retina ”

“ A single layer of tissue-paper, gummed on to the out-side of the tube, and turned down about one sixteenth of an inch all around the edge of the two-inch lens, and around the disk at the other end, will now serve as a sort of fastener to both, and will complete the eye-piece, which is shown in full size in section, Fig. 3. The smaller lens a must be next the eye when the telescope is fitted up; the larger lens b, called the field-glass, will be inside and facing the object-glass. ”

“ In the eyeball are first a crystalline lens, next an aqueous humor, third a vitreous humor; these three so vary in their qualities of refraction and dispersion as to render images quite free from color fringes. Compound lenses on the Dollond principle, repeating the structure of an eyeball, are used in all good telescopes, microscopes, and cameras, and are now executed in varieties of Jena glass which bring perturbing hues to the vanishing point. In their achromatic, or color-free, lenses and their cameras, or dark chambers, our photographic instruments much resemble the eye. ”

“ 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. ”

“ To unite the rays near G or H the angle of the flint prism must be diminished; that is, the focal length of the flint lens must be lengthened as compared with that of an object-glass of similar construction suited for eye observations; and the rays of greatest photographic action can be united more perfectly than the visible rays.
Triple object-glasses.If an object-glass is composed of three lenses of different kinds of glass it is theoretically possible to unite three instead of two points of the spectrum, besides improving the correction for spherical aberration. ”

“ The crystalline lens consists of infinite numbers of extremely thin transparent little plates, each of which is in itself composed of fine fibres so united together as to be capable of a small degree of compression or extension. Hence the power of the lens to alter its form according to circumstances. It is calculated that the human eye contains over five millions of the laminæ above referred to. With such wonders is the world of nature replete,—wonders that we daily and hourly pass by without examination. ”

“ First of all a single lens saves about 10% of the light. Each surface of glass through which light passes transmits 95 to 96% of that light, so that a single lens transmits approximately 90%, two lenses 81% and so on. This loss may be enough to determine the visibility of an object. Sir Wm. Herschel found that faint objects invisible with the ordinary two lens eyepiece came to view with the single lens. ”

“ Is it not curious, that so vast a being as the whale should see the world through so small an eye, and hear the thunder through an ear which is smaller than a hare’s? But if his eyes were broad as the lens of Herschel’s great telescope; and his ears capacious as the porches of cathedrals; would that make him any longer of sight, or sharper of hearing? Not at all.—Why then do you try to “enlarge” your mind? Subtilize it. ”

“ 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. ”

“ Note that in hypermetropia and myopia the rays, instead of coming to a focus, form a round spot upon the retina. The interference of the lens, however, is believed to account for only moderate degrees of variation in errors of refraction, and that only during the earlier years of life. For the higher ones, or those that occur after fortyfive years of age, when the lens is supposed to have lost its elasticity to a greater or less degree, no plausible explanation has ever been devised. ”

“ Now those colours have become lately of the greatest value to astronomers, but at first they were merely a nuisance; they interfered with the use of lenses in telescopes. A lens is a piece of glass with curved surfaces; when rays of light strike those surfaces they are bent to a focus (Fig. 27) , as is wanted for making a telescope. But the bending introduces colour which is not wanted; and yet they could not in early days get rid of the colours without also getting rid of the bending to focus. ”

“ The image, however, is turned upside down, because the light which comes from the upper part of the luminous object and goes through the lens, passes downwards towards the lower part of the screen KL. These properties of a lens can easily be proved by experiments with a common burning glass, or a reading glass, or spectacle glass, such as is used by elderly people. ”

“ The great obstacle to viewing small objects at the usual distance, arises from too great a divergence of the light reflected from them, which causes the rays to reach the retina before they have converged to a focus. This defect is remedied by convex lenses, which bring the visual point nearer to the eye, and consequently cause the rays to concentrate in a large focus upon the retina. The most powerful microscopic lenses are small globules of glass, which permit the eye to be brought very near to the object. ”

“ 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. ”

“ 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. ”

“ When a revolving light has to be made by the dioptric apparatus, the lenses are so constructed that the light, in going through them, is gathered up into the exact similitude of a ray, as it would leave the mouth of a reflector, and of course with the same result; the central lamp remains stationary, and the lenses move round it, and focus after focus, flash after flash, come upon the eye of the mariner. ”

“ The outer lens of the objective, designated the ‘front lens,’ is in form a plano-convex lens. The inner lens, called the ‘back lens,’ consists of two separate lenses, one an unequi-convex lens, the other a meniscus-concave lens. This combination gives a beautifully-defined picture on the screen, with flatness of field and abundance of light.
The lamp is entirely distinct from the body, and will burn equally well either outside or inside the body of the lantern, the chimney being attached to the lamp and not to the body. The cistern of the lamp forms its base, and is filled with mineral oil. ”

“ 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. ”

“ We next study the utilization of this power of refraction in the manufacture of lenses to overcome the unequal refrangibility of the colored rays of light.
A lens for use in photography is made of glass as pure and as colorless as can be procured, and is ground into such a form as to collect or disperse the rays of light which pass through it. Lenses are of different shapes, and thence receive different names. p88
FIG. 2 E.
1. Double convex. 2. Plano-convex. 3. Concavo-convex.
4. Double concave. 5. Plano-concave. 6. Meniscus. ”