Focus (optics)

“ It will be readily seen that a spherical mirror may have an infinite number of conjugate foci, according to the distance of the source of light. It is also clear, that if we cause the light to approach the mirror, the focus will also approach it. ”

“ I myself denounce the practice of combining glasses together, in all those cases where they are capable of doing their work alone. I shall always consider it as a clumsy, bungling, and unworkmanlike method of obtaining a short focus, combined [p254] with a large angle of aperture. If a man aims at perfection, and wishes to distinguish himself in this branch of optics, let it be done by working perfect triple glasses of 0.2 and 0.3 inch focus, with 0.1 and 0.15 of perfect aperture, like those of Mr. W. Tulley and Mr. ”

“ Of all the senses none is more frequently the seat of such deceptive judgments than that of sight; and in the science of physiological optics a very considerable share of attention is claimed by optical illusions. For the purposes of convenience, we may draw a distinction between these illusions, which are the direct result of certain properties or imperfections of the eye as an optical instrument, and those which arise from obliquities of judgment in interpreting the sensations optically impressed upon the retina of the eye. ”

“ Thus, that there should be eyes capable of seeing, the laws of optics must be satisfied, reflection, refraction and the rest, just as exactly in the making of an eye as in that of a telescope. De facto they are satisfied. ”

“ The peculiar appearances known as spectres in optics are certain illusions of vision in which an object is apparently presented to the view which does not really exist. In such cases either the brain, the retina, or the optic nerve are unnaturally excited, and made sensitive to an appearance that, physically speaking, does not exist. There is such a close connexion between the senses and the mind, that we continually, and without knowing it, transfer to the physical world that which belongs to the domain of thought. ”

“ All these slight imperfections make the eye’s visions more imperfect than that of the optician’s lens, even when objects in one plane only are sharply focussed, therefore, except in very rare cases, which will be touched upon elsewhere, the chief point of interest should be slightly—very slightly—out of focus, while all things, out of the plane of the principal object, it is perfectly obvious, from what has been said, should also be slightly out of focus, not to the extent of producing destruction of structure or fuzziness, but sufficiently to keep them back and in place. ”

“ The pencil of light is not only turned aside, but it is also widened out into a band which is truly painted with all the colours of the rainbow, every tone and hue being of the most marvellous brilliancy. This long coloured stripe, which constitutes one of the most beautiful sights that the science of optics can afford us, is known to scientific men by the name of the solar spectrum. ”

“ By inspecting a dead carcass we shall never discover the principle of life; by inspecting a dead eye, or a camera obscura, we shall never discover the principle of vision. Yet, though there is no seeing in a dead eye, or in a camera obscura, optics deal exclusively with such inanimate materials; and hence the student who studies them will do well to remember, that optics are the science of vision, with the "fact" of vision left entirely out of the consideration. ”

“ Chalmers seated among his friends on the Moderator’s chair––we find an exemplification sufficiently striking of the laws on which this seemingly mysterious power depends. They are purely structural laws, and relate not to the mind, but to the eye,––not to the province of the metaphysician, but to that of the professor of optics. The lens of the camera obscura transmits the figures to the prepared paper, on quite the same principle on which in vision the crystalline lens conveys them to the retina. ”

“ When the focusing apparatus of the eye is not perfect, it can be made so by putting in front of the eye the proper kind of lens. There is nothing strange or mysterious about it. In some cases, the eye focuses the light before it reaches the retina. Such cases are known as nearsightedness and are corrected by having placed in front of the eyes concave lenses of the proper strength. These lenses diverge the rays and make them focus on the retina. ”

“ The human eye, exquisitely constructed as it is, is nevertheless an imperfect mechanism, being limited in its action. It can only perceive waves of a definite length, as its fellow organ, the ear, can only distinguish a limited range of sounds. It can only take note of vibrations that are transverse, as the ear can only take note of those that are normal. In optics there are two distinct orders of facts; the actual relations of light itself, and the physiological relations of our organ of vision, with all its limitations and imperfections. Light is altogether the creation of the mind. ”

“ 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. ”

“ Let us suppose that we are looking at a coloured object—let us say, a red railway signal-lamp; from the lamp to our eye there flows a continuous line of luminous undulations; these undulations enter the eye and become depicted on the retina. For every wave that passes through the pupil, there is a separate and corresponding vibration of the optic nerve, and the number of these vibrations that take place in the course of a second can be easily calculated if we know the velocity of light and the breadth of the waves. We have before found that light travels at the rate of 185,000 miles per second ”

“ They are: 'optical refraction', 'light-ray', and 'light-velocity' - the latter two serving to explain the first. In a science of optics which seeks its foundation in the intercourse between man's own visual activity and the doings and sufferings of light, these three concepts must needs undergo a decisive change, both in their meaning and in their value for the description of the relevant optical phenomena. For they are all purely kinematic concepts typical of the onlooker-way of conceiving things - concepts, that is, to which nothing corresponds in the realm of the actual phenomena. ”

“ The lens in the eye can change its shape a little, so that it will focus objects a mile off, or ten inches off, but it cannot be pushed in and out like the tube in your camera. You can do this, however, if you take another lens and hold it outside your eye and let the light go through that first before it comes to the lens in your eye, and in this way you can get a focus in your retina, and the outside lens thus forms a part of that optical instrument called your eye. ”

“ Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark preservers are often in the habit of observing all that is going on behind their backs by the reflection seen in the corner of their glasses.
Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. ”

“ To gain a clear insight into the mode in which a single lens serves to magnify objects, it will be necessary to revert to the phenomena of ordinary vision. An eye free from any defect has a considerable power of adjusting itself to very considerable distances. One of the special functions of the eye is bringing the rays of light, by a series of dioptric mechanisms, to a perfect focus on its nervous sensitive layer, the retina. ”

“ The object to be examined is placed on a little shelf projecting from the stand, half an inch below the tube, and reduced to the nearer focus by a plain mirror, placed at half a right angle to the axis of the speculum, at the distance of 11/2 inch from it. The image, formed in the farther focus, is viewed through a lens, which may be changed at pleasure, so as to increase or diminish the magnifying power, the object remaining unmoved, which gives this instrument a great advantage, in point of convenience, over the common compound refracting microscope. ”

“ Let us hope, however, that the science of optics has now become so familiar to most educated people, that no such thing as a real imposition can take place, although at the present day there are so many exhibitions of the marvellous that ordinary observers have the greatest difficulty in accounting for them. ”

“ The foci of optics are proofs of the illusion of what is termed material sense, as when sky and tree-tops appear to join hands, and clouds and ocean to meet and mingle, when they are as remote at the farther as at the nearer point of vision. ”

“ By and by, we shall learn both the length and rapidity of these vibrations, but it will be as well first to describe the experiments made in this direction by the immortal Newton himself.
Every one has, doubtless, at one period of his life, amused himself with blowing soap-bubbles by means of a tobacco-pipe and a little lather—a sufficiently childish amusement, you will possibly say, but one narrowly connected with the most intricate secrets of the science of optics. ”

“ Not only this: the spot being always situated on the outer and indistinct portion of the image reflected on the retina, we do not take notice of it; for as every one has no doubt observed, it is only the small portion of the object we are looking at exactly opposite the centre of the eye that is perfectly distinct and clear, all the rest being confused in its details, although quite visible. ”

“ But as we are able to form a distinct image of near objects, and as we notice when we turn our gaze from far to near objects there is a distinct feeling of muscular effort in the eyes, there must be some means whereby the eye can readily adapt itself for focussing near and distant objects. In a photographic camera the focus can be readily altered, either by changing the lenses, employing a lens of greater or less curvature, or by altering the distance of the screen from the lens. ”

“ The faculty of seeing stereoscopic slides single without an instrument is of the utmost utility to the student of physiological optics, and persons with strong eyes can easily acquire it. The only difficulty lies in dissociating the degree of accommodation from the degree of convergence which it usually accompanies. If the right picture is focussed by the right eye, the left by the left eye, the optic axes must either be parallel or converge upon an imaginary point some distance behind the plane of the pictures, according to the size and distance apart of the pictures. ”

“ Writing on Optics, he says: “’Tis pleasant to undeceive the eye in the common accidents of life, and to see it approach, in some measure, towards that certainty of judging and apprehending visibles that it will attain to at the day of resurrection, when it will be above the power of being cheated by concave or convex, or deluded by a refraction or reflection. This may, in a great measure, be accomplished in this world by such as give themselves up to the study of optics.” ”

“ Every point upon the surface of her clothes and face is reflected back to her eye from the surface of the tin amalgam which has been applied to the back of the mirror by the looking-glass maker, for the purpose of rendering the image of the object more brilliant than if the glass alone were used. The rays which proceed from every one of these points strike upon the surface of this metallic layer, are stopped by its opacity, and are reflected back to the eye at an angle equal to that at which they strike the surface. ”

“ The phenomena of ocular spectra and complementary colours experienced by every one forms a curious chapter in the history of those illusions which take their origin in the eye itself. Every one has noticed that after looking fixedly at a bright light or a striking colour for a few moments, the eye preserves an impression of the object for a certain time. A very light window looked at intently for several seconds will leave the impression of its cross-bars on the retina for several minutes, the colour of the image changing at every movement of the eye. ”

“ The opening in the centre, which appears like a black spot when the eye is examined, is not really an object, but simply an aperture, capable of changing its size according to the quantity of light striking the eye. ”

“ If the rays are very convergent, as those proceeding from a convex lens and coming to a focus, the lens of a normal eye will bring them to a focus far in front of the retina, as if the person were very short-sighted. ”

“ Thus we see that an image is formed on both eyes, and we consequently see the object, whatever it may be, repeated twice. By degrees, however, the eyes gain the power of converging their axes on objects at different distances, so that they fall on similar portions of each retina, and so convey a single impression to the brain. ”