[Sidenote: 69. Sensitiveness of all parts of the retina? Experiment to prove the existence of the "blind spot."]

69. All parts of the retina are not equally sensitive, and singularly enough, the point of entry of the nerve of sight, in the back part of the eyeball, is entirely insensible to light, and is called the "blind spot."

The existence of this point may be proven by a simple experiment. Hold the accompanying figure, on page 207, directly in front of and parallel with the eyes. Close the left eye, and fix the sight steadily on the left-hand circle; then, by gradually varying the distance of the figure from the eye, at a certain distance (about six inches), the right-hand circle will disappear, {208} but nearer or further than that, it will be plainly seen.

The other eye may be also tried, with a similar result: if the gaze be directed to the right-hand circle, the left one will seem to disappear. The experiment may be repeated by using two black b.u.t.tons on the marble top of a bureau, or on some other white surface. The blind spot does not practically interfere with vision, since the eye is seldom fixed immovably on an object, and the insensitive parts of the two eyes can never be directed upon the same object at the same time.

[Sidenote: 70. Duration of impressions upon the retina? How ill.u.s.trated?]

70. Impressions made upon the retina are not at once lost, but persist a measurable length of time, and then gradually fade away. Thus, a bright light or color, gazed at intently, cannot be immediately dismissed from sight by closing or turning away the eyes. A stick lighted at one end, if whirled around rapidly in the dark, presents the appearance of an unbroken luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to be merged into a plane surface. If an object move too rapidly to produce this sort of lasting impression, it is invisible, as in the case of a cannon-ball pa.s.sing through the air in front of us.

[Sidenote: 71. What further ill.u.s.tration? Winking, why it is not noticed.

Ease with which the retina is fatigued or deprived of sensibility? How shown?]

71. If a card, painted with two primary colors--as red and yellow--be made to rotate swiftly, the eye perceives neither of them distinctly; but the card appears painted with their secondary color--orange. The average duration of retinal images is estimated at one-eighth of a second; and it is because they thus endure, that the act of winking, which takes place so frequently, but so quickly, is not noticed and does not interrupt the vision. The retina is easily fatigued or deprived of its sensibility. After looking steadfastly at a bright light, or at a white object on a black ground, a dark spot, corresponding in shape to the bright object, {209} presents itself in whatever direction we look. This spot pa.s.ses away as the retina resumes its activity.

[Sidenote: 72. How further shown? How is the result accounted for?

"Color-blindness?"]

72. If a bright color be gazed at intently, and the eyes then be turned to a white surface, a spot will appear; but its color will be the complement of that of the object. Fix the eye upon a red wafer upon a white ground, and on removing the wafer a greenish spot of the same shape takes its place. This result happens because a certain portion of the retina has exhausted its power to perceive the red ray, and perceives only its complementary ray, which is green. The color thus subst.i.tuted by the exhausted retina is called a physiological or accidental color. In some persons the retina is incapable of distinguishing different colors, when they are said to be affected with "color-blindness." Thus, red and green may appear alike, and then a cherry-tree, full of ripe fruit, will seem of the same color in every part. Railroad accidents have occurred because the engineer of the train, who was color-blind, has mistaken the color of a signal.

[Sidenote: 73. The location of the crystalline lens? How supported? Its color and texture? Shape? Size?]

73. THE CRYSTALLINE LENS.--Across the front of the eye, just behind the iris, is situated the _Crystalline lens_, enclosed within its own capsule.

It is supported in its place partly by a delicate circular ligament, and partly by the pressure of adjacent structures. It is colorless and perfectly transparent, and has a firm but elastic texture. In shape it is doubly convex, and may be rudely compared to a small lemon-drop. The front face of the lens is flatter than the other, and is in contact with the iris near its pupillary margin, as is represented in the diagram on page 214. It is only one-fourth of an inch thick.

[Sidenote: 74. Cataract? Aqueous humor? Vitreous humor?]

74. When this little body becomes opaque, and no longer affords free pa.s.sage to the rays of light, as often happens {210} with the advance of age, an affection termed "cataract" is produced. Between the crystalline lens and the cornea is a small s.p.a.ce which contains the _aqueous humor_ (see Fig. 48, A). This humor consists of five or six drops of a clear, colorless liquid very much like water, as its name implies. That part of the globe of the eye lying behind the lens is occupied by the _vitreous humor_, so called from its fancied resemblance to melted gla.s.s (Fig. 48, V). This humor is a transparent, jelly-like ma.s.s, enclosed within an exceedingly thin membrane. It lies very closely applied to the retina, or nervous membrane of the eye, and const.i.tutes fully two-thirds of the bulk of the eyeball.

[Ill.u.s.tration: FIG. 50.--THE RETINAL IMAGE.]

[Sidenote: 75. What is a lens and its focus? The miniature image, how produced?]

75. THE USES OF THE CRYSTALLINE LENS.--A convex lens has the property of converging the rays of light which pa.s.s through it; and the point at which it causes them to meet is termed its focus. If a lens of this description, such as a magnifying or burning-gla.s.s, be held in front of an open window, in such a position as to allow its focus to fall upon a piece of paper, it will be found to depict upon the paper a miniature image of the scene outside of the window. It will be further noticed that the image is inverted, or upside down, and that the paper {211} at the place upon which the image is thrown is much brighter than any other part.

[Sidenote: 76. How are figures painted upon the retina? How proved?]

76. Now all the transparent structures of the eye, but especially the crystalline lens, operate upon its posterior part, or retina, as the convex lens acts upon the paper; that is, they paint upon the retina a bright inverted miniature of the objects that appear in front of the eye (Fig.

50). That this actually takes place may be proved by experiment. If the eyeball of a white rabbit, the walls of which are transparent, be examined while a lighted candle is held before the cornea, an image of the candle-flame may be seen upon the retina.

[Sidenote: 77. What can be said in respect to the form and structure of the crystalline lens?]

77. The form and structure of the crystalline lens endow it with a remarkable degree of refractive power, and enable it to converge all the rays of light that enter it through the pupil, to a focus exactly at the surface of the retina. When this lens is removed from the eye, as is frequently done for the cure of cataract, it is found that the rays of light then have their focus three-eighths of an inch behind the retina; that the image is four times larger than in the healthy eye, that it is less brilliant, and that its outline is very indistinct. From this we learn that one of the uses of the crystalline lens is to make the retinal image bright and sharply-defined, at the same time that it reduces its size.

Indeed, the small size of the image is a great advantage, as it enables the limited surface of the retina to receive, at a glance, impressions from a considerable field of vision.

[Sidenote: 78. How is the inverted image upon the retina presented in its true position to the mind?]

78. As the image upon the retina is inverted, how does the mind perceive the object in its true, erect position? Many explanations have been advanced, but the simplest and most satisfactory appears to be found in the fact that {212} the retina observes no difference, so to speak, between the right and left or the upper and lower positions of objects. In fact, the mind is never conscious of the formation of a retinal image, and until instructed, has no knowledge that it exists. Consequently, our knowledge of the relative location of external objects must be obtained from some other source than the retina. The probable source of this knowledge is the habitual comparison of those objects with the position of our own bodies: thus, to see an elevated object, we know we must raise the head and eyes; and to see one at our right hand, we must turn the head and eyes to the right.

[Ill.u.s.tration: FIG. 51.--THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE.

N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye.

S, Parallel Rays from the Sun.]

[Sidenote: 79. The uniform perfection of the eye? Examples? The most common imperfection?]

79. LONG-SIGHT OR HYPEROPIA, AND SHORT-SIGHT OR MYOPIA.--The eye is not in all cases perfectly formed. For example, persons may from birth have the cornea too prominent or too flat, or the lens may be too thick or too thin.

In either of these conditions sight will be more or less defective from the first, and the defect will not tend to disappear as life advances. The most common imperfection, however, is in the shape of the globe; which may be short (Fig. 51, H), as compared with the natural eye, N, or it may be too long, M.

[Sidenote: 80. How is "long-sight" explained? "Short-sight?"]

80. When the globe is short, objects can only be clearly {213} seen that are at a distance, and the condition of the vision is known as "long-sight," or hyperopia. It will be observed, by reference to Fig. 51, that the focus of the rays of light would fall behind the retina of this eye. When the globe is too long, objects can only be clearly seen that are very near to the eye; and the condition resulting from this defect is termed "short-sight," or myopia. The focus of the rays of light is, in this case, formed in the interior of the eye in front of the retina.

[Sidenote: 81. Long-sight, how common? With what must it not be confounded?

Kind of gla.s.ses for short-sight? Why? Squint?]

81. Long-sight, or hyperopia, is common among schoolchildren, nearly as much so as short-sight, and must not be confounded with the defect known as the "far sight" of old people; although in both affections the sight is improved by the use of convex gla.s.ses. Children not infrequently discover that they see much better when they chance to put on the spectacles of old persons. For the relief of short-sight, concave gla.s.ses should be employed; as they so scatter the rays of light as to bring the focus to the retina, and thus cause the vision of remote objects to become at once distinct.

That form of "squint," in which the eyes are turned inward, is generally dependent upon long-sight, while that rarer form, when they turn outward, is due to short-sight.

[Sidenote: 82. What is stated in connection with the opera-gla.s.s?

Experiment with pencil and distant object?]

82. THE FUNCTION OF ACCOMMODATION.--If, after looking through an opera-gla.s.s at a very distant object, it is desired to view another nearer at hand, it will be found impossible to obtain a clear vision of the second object unless the adjustment of the instrument is altered; which is effected by means of the screw. If an object, like the end of a pencil, be held near the eye, in a line with another object at the other side of the room, or out of the window, and the eye be fixed first upon one and then upon the other, it will be found that when the pencil is clearly seen, the {214} further object is indistinct; and when the latter is seen clearly, the pencil appears indistinct; and that it is impossible to see both clearly at the same time. Accordingly, the eye must have the capacity of adjusting itself to distances, which is in some manner comparable to the action of the screw of the opera-gla.s.s.

[Ill.u.s.tration: FIG. 52.--THE FUNCTION OF ACCOMMODATION.

The right half of the diagram shows the eye at rest. The left half shows the lens accommodated for near vision.]

[Sidenote: 83. Function of accommodation? In what does it consist? How is the function explained?]

83. This, which has been called the function of accommodation, is one of the most admirable of all the powers of the eye, and is exercised by the crystalline lens. It consists essentially in a change in the curvature of the front surface of the lens, partly through its own elasticity, and partly through the action of the ciliary muscle. When the eye is at rest, that is, when accommodated for a distant object, the lens is flatter and its curvature diminished (see Fig. 52); but when strongly accommodated for near vision the lens becomes thicker, its curvature increases, and the image on the retina is made more sharp and distinct. Since a strong light is not required in viewing near objects, the pupil contracts, as is shown in the left-hand half of the diagram. {215}

[Sidenote: 84. Change of sight with the approach of old age? Explain the change?]

84. OLD-SIGHT, OR PRESBYOPIA.--But this marvellously beautiful mechanism becomes worn with use; or, more strictly speaking, the lens, like other structures of the body, becomes harder with the approach of old age. The material composing the lens becomes less elastic, the power to increase its curvature is gradually lost, and as a consequence, the person is obliged to hold the book further away when reading, and to seek a stronger light. In a word, the function of accommodation begins to fail, and is about the first evidence that marks the decline of life. By looking at the last preceding diagram, and remembering that the increased curvature of the lens cannot take place, it will be at once understood why old-sight is benefitted in near vision by the convex lens, such as the spectacles of old people contain. It acts as a subst.i.tute for the deficiency of the crystalline lens.

[Sidenote: 85. Hearing or audition? What is sound? How propagated commonly?

Stone thrown in water?]

85. THE SENSE OF HEARING.--SOUND.--Hearing, or audition, is the special sense by means of which we are made acquainted with _sound_. What is sound?

It is an impression made upon the organs of hearing, by the vibrations of elastic bodies. This impression is commonly propagated by means of the air, which is thrown into delicate undulations, in all directions from the vibrating substance. When a stone is thrown into smooth water, a wave of circular form is set in motion, from the point where the stone struck, which constantly increases in size and diminishes in force, as it advances.

[Sidenote: 86. Sound-wave in the atmosphere? Its shape? Rate of motion?

Sound, in water, air, and solid bodies?]

86. Somewhat resembling this, is the undulation, or sound-wave, which is imparted by a sonorous vibration to the surrounding atmosphere. Its shape, however, is spherical, rather than circular, since it radiates upward, downward, and obliquely as well as horizontally, like the wave {216} in water. The rate of motion of this spherical wave of air is about 1050 feet per second, or one mile in five seconds. In water, sound travels four times as fast as in air, and still more rapidly through solid bodies; along an iron rod, its velocity is equal to two miles per second.

[Sidenote: 87. The earth as a conductor of sound? To what has the western Indian been taught? Solid substances as conductors? As regards sound, in what respect is air necessary? Sound in a vacuum?]

87. The earth, likewise, is a good conductor of sound. It is said that the Indian of our western prairies can, by listening at the surface of the ground, hear the advance of a troop of cavalry, while they are still out of sight, and can even discriminate between their tread and that of a herd of buffaloes. Solid substances also convey sounds with greater power than air.