Sight (optics)

“ Op′tic (Pope) , an organ of sight: an eye.—adv. Op′tically.—ns. Optic′ian, one skilled in optics: one who makes or sells optical instruments; Op′tics (sing.) , the science of the nature and laws of vision and light; Optim′eter, Optom′eter, an instrument for measuring the refractive powers of the eye; Optom′etry, the measurement of the visual powers.—Optic axis, the axis of the eye—that is, a line going through the middle of the pupil and the centre 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. ”

“ 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. ”

“ 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. ”

“ For normal eyes the natural adaptation is not to focus for quite parallel rays, but on objects at a moderate distance, and practically, therefore, most persons do adjust the focus of a telescope, for most distinct and easy vision, so that the rays emerge from the eye-piece very slightly divergent. Abnormally short-sighted persons require to push in the eye-lens nearer to the object-glass, and long-sighted persons to withdraw it from the adjustment employed by those of normal sight. ”

“ 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 ”

“ 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. ”

“ A lamplight, for example, sets in motion waves of ether, which pass in through the pupil of the eye, to the retina, where the rods and cones transmit the vibration through the optic nerve to the brain, and then the mind perceives the light. (Note, p. 236.)
THE USE OF THE CRYSTALLINE LENS. [Footnote: The uses of the eye and ear are dependent upon the principles of Optics and Acoustics. They are therefore best treated in Physics.] —A convex lens, as a common burning glass, bends the rays of light which pass through it, so that they meet at a point called the focus. ”

“ 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. ”

“ 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. ”

“ The rays of light A B are first reflected from the concave mirror M on to the surface of the small plane mirror m, which is placed at an angle of 45°, and reflects them as far as the point A' B', where they form the image to be magnified by the eye-glass. It is therefore at the side of the instrument, and not at the end, as hitherto, that the observer is placed, and at right angles to the path of the rays. ”

“ 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. ”

“ When merely common lenses are used in this telescope, the eye glass is limited in power by the necessary confusion in the first image from the aberrations, which become very sensible when that image is much magnified, particularly those arising from the unequal refraction of the light. ”

“ 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. ”

“ We all know the distortion that our face undergoes when reflected from the shining surface of a teapot or spoon, and the cylindrical mirrors that hang in the shop windows of many opticians are the source of much amusement to the passers by, whose physiognomies are shown to them either lengthened to many times their natural size, or widened to an extent that is ludicrously hide-ous, according to the position in which the mirror is hung. ”

“ The retinoscope is an instrument used to measure the refraction of the eye. It throws a beam of light into the pupil by reflection from a mirror, the light being either outside the instrument—above and behind the subject—or arranged within it by means of an electric battery. On looking through the sight-hole one sees a larger or smaller part of the pupil filled with light, which in normal human eyes is a reddish yellow, because this is the color of the retina, but which is green in a cat's eye, and might be white if the retina were diseased. ”

“ We know by experience that when we look at an object and press one of the eyeballs slightly with the finger, the image of it becomes doubled. The explanation of this phenomenon is not very easy, but it is generally supposed that in the case of ordinary vision the two eyes produce the sensation of a single image in consequence of the two impressions being formed at corresponding parts of each retina, and that habit causes us to see only a single object in such a case. ”

“ If, therefore, we can by means of lenses or prisms cause the image of any natural object, as seen by the right eye, to be conveyed to the left, and vice versâ, we shall see the relief reversed. A conical cap will appear in relief as a cone, a globe will look like a hollow sphere, and the human face will take the semblance of the inside of a mask. The same deception may be effected by looking at a seal through a short-focused lens, so that the image shall seem reversed. ”

“ In showing ordinary observers an object in the telescope, it is well to insist on their moving the eye-piece backwards and forwards until distinct vision is obtained, for it often happens that people will say they see an object quite distinctly, when it is in reality misty, and will generally refuse to allow the focus to be altered. ”

“ But it, after we have got the object glass properly squared to the axis of the tube or the line of sight, the image and the ring system in and out of focus still appear oblong, the fault of astigmatism must exist either in the objective, the eyepiece, or the eye. The chances are very great that it is the eye itself that is at fault. ”

“ The beginning of optics [6] will be to look up at the light, a thing natural to children; for the instant it becomes visible, they turn their eyes to it. They must, however, be watched, and not be permitted to look with fixed eyes on excessive light and brilliance, strongly affecting the power of vision, especially at first, lest that power be weakened, or extinguished by overstraining. Let them have the means of seeing moderate light, especially of a green color, and gradually anything that shines. ”

“ The persistence of luminous impressions upon the retina has given rise to the invention of a number of well-known optical toys, amongst which may be mentioned the phenakistiscope, the thaumatrope, the phantascope, and many others. ”

“ 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. ”

“ It is not necessary to be thoroughly acquainted with the science of optics in order to see. A man may look through a telescope before he can define the refraction or reflection of light. Now all that is included in the word salvation hangs on this simple condition. ”

“ 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. ”

“ But when the eyes are so disposed as to be capable of seeing distant objects distinctly, the two images formed by a near object are no longer found in the corresponding portions of each retina, and so produce the sensation of double vision. The same thing happens when either of the eyes is momentarily displaced.
These phenomena have given rise to the construction of a very simple instrument, the phantascope, with which many interesting experiments may be performed, and which was invented some years since by Dr. ”

“ 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. ”

“ We shall, therefore, in the present chapter, speak of an ingenious instrument which serves to show in relief the images of objects depicted on a flat surface. We have already seen, that although we have two eyes, provided with lenses and screens by means of which the images of things around us are formed, we only perceive a single object; and the student has no doubt long since wondered why nature has bestowed two eyes upon us, when only one would have apparently served the same purpose. ”

“ Such a machine requires an intelligent manufacturer; and yet we have only as yet been looking at the dead eye, paying no regard to sight at all. Even a blind man's eye prove an intelligent Creator.
Let us now turn our thoughts to the instrument of sight. The optic nerve is the part of the eye which conveys visions to the mind. ”