Chapter XI: The Special Senses (2)

Various mental emotions, as joy and grief, may produce similar results. In these cases the glands secrete the fluid in such quantities that it cannot escape by the lacrymal canals, and the excess rolls over the cheeks as tears. Excessive grief sometimes acts on the nerve centers in exactly the opposite manner, so that the activity of the glands is arrested and less fluid is secreted. This explains why some people do not shed tears in times of deep grief.

Experiment 155. Gently turn the inner part of your lower eyelid down. Look in a mirror, and the small lacrymal point, or opening into the nasal duct, may be observed.

337. Color-blindness. There is an abnormal condition of vision called color-blindness, in which the power of discrimination between different colors is impaired. Experiment shows that ninety-six out of every one hundred men agree as to the identity or the difference of color, while the remaining four show a defective perception of color.

The first may be said to have _normal vision_; the second are called _color-blind_. It is a curious fact that ten times more men than women are color-blind.

In its true sense, color-blindness is always congenital, often hereditary. This condition of abnormal vision is totally incurable. A person may be color-blind and not know it until the defect is accidentally revealed. The common form of defective color-vision is the inability to distinguish between _red_ and _green_. As green lights mean safety, and red lights danger, on railroads, on shipboard, and elsewhere, it becomes of paramount importance that no one who is color-blind should be employed in such service. Various tests are now required by statute law in many states to be used for the detection of such defects of vision among employees in certain occupations.

338. School Life and the Eyesight. The eyes of children need more care than those of adults, because their eyes are still in the course of development. The eyes, like any other organ which is yet to attain its full growth, require more care in their use than one which has already reached its full size. They are peculiarly liable to be affected by improper or defective light. Hence the care of the eyes during school life is a matter of the most practical importance.

In no matter of health can the teacher do a more distinct service than in looking after the eyesight of the pupils. Children suffering from defective vision are sometimes punished by teachers for supposed stupidity. Such pupils, as well as the deaf, are peculiarly sensitive to their defects. Every schoolroom should have plenty of light; it should come from either side or the rear, and should be regulated with suitable shades and curtains.

Pupils should not be allowed to form the bad habit of reading with the book held close to the eyes. The long search on maps for obscure names printed in letters of bad and trying type should be discouraged. Straining the eyes in trying to read from slates and blackboards, in the last hour of the afternoon session, or in cloudy weather, may do a lifelong injury to the eyesight. Avoid the use, so far as possible, especially in a defective light, of text-books which are printed on battered type and worn plates.

The seat and desk of each scholar should be carefully arranged to suit the eyesight, as well as the bones and muscles. Special pains should be taken with the near-sighted pupils, and those who return to school after an attack of scarlet fever, measles, or diphtheria.

Experiment 156. _To test color-blindness._ On no account is the person being tested to be asked to name a color. In a large class of students one is pretty sure to find some who are more or less color-blind. The common defects are for red and green.

Place worsteds on a white background in a good light. Select, as a test color, a skein of light green color, such as would be obtained by mixing a pure green with white. Ask the examinee to select and pick out from the heap all those skeins which appear to him to be of the same color, whether of lighter or darker shades. A color-blind person will select amongst others some of the confusion-colors, _e.g._, pink, yellow. A colored plate showing these should be hung up in the room. Any one who selects all the greens and no confusion-colors has normal color vision. If, however, one or more confusion-colors be selected, proceed as follows: select as a test color a skein of pale rose. If the person be red-blind, he will choose blue and violet; if green-blind, gray and green.

Select a bright red skein. The red-blind will select green and brown; the green-blind picks out reds or lighter brown.

339. Practical Hints on the Care of the Eyes. The eye is an exceedingly delicate and sensitive organ. While it is long-suffering, its endurance has a limit. Like all the other organs of the body, the eyes are better for moderate and rational use. More than any other organ they require attention to the general health, as the condition of the skin, exercise in the open air, good food, and proper habits of daily living.

The tissues of the eyes are peculiarly sensitive to any general influence. Certain constitutional diseases, like rheumatism, lead-poisoning, diphtheria, and measles often affect the eyes. Special care should be taken with children’s eyes during and after an attack of measles and scarlet fever. The eyes of young infants should not be exposed to glaring lights or to the direct rays of the sun, as when taken out in baby carriages.

Illustration: Fig. 136.—Showing the Relative Position of the Lacrymal Apparatus, the Eyeball, and the Eyelids.

A, lacrymal canals, with the minute orifices represented as two black
dots (puncta lacrymalia) to the right;
B, tendon of the orbicularis palpebrarum muscle; apparently under B
is seen the lacrymal sac. The minute openings of the Meibomian glands
are seen on the free margins of the eyelids.

Below A is seen a small conical elevation, with black dots (the lacrymal papilla or caruncle).

Glasses should be worn when they are needed. A failure to do this ususally causes much unnecessary suffering. It is far from wise to postpone as long as possible the first use of glasses. The selection and proper fitting of glasses call for the combined skill of both the physician and the optician. Obstinate headaches are often caused by defective vision, and may disappear after discontinuing improper glasses.

The habit of reading, in the cars or elsewhere, the daily paper and poorly printed books, with their blurred and indistinct type, is a severe strain on the accommodation apparatus of the eyes. It is a dangerous practice to read in bed at night, or while lying down in a darkened or shaded room. This is especially true during recovery from illness. The muscles of the eyes undergo excessive strain in accommodating themselves to the unnatural position. The battered type, wood-pulp paper, and poor presswork, now so commonly used in the cheap editions of books and periodicals, are often injurious to the eyesight.

Reading-matter should not be held nearer to the eyes than is necessary to make the print appear perfectly sharp and distinct. No print should be read continuously that cannot be seen clearly at about eighteen inches. Those who read music are especially liable to strain the eyes, because exact vision is required to follow the notes. Persons who wear glasses for reading should be careful to use them while reading music, and good light is necessary to avoid any undue strain.

After reading steadily for some time, the eyes should be rested by closing them a short period or by looking at some distant object, even if only for a few moments. The book, the sewing, and work generally, should be held as far from the eyes as is compatible with good vision. The natural tendency is to reverse this rule. We should never read, write, sew, stitch, or otherwise use the eyes when they smart or tingle, or when the sight is dim or blurred. The eyes are then tired and need a rest. Much injury may be done by reading in twilight, or by artificial light in the early morning, and by reading and working in badly lighted and ill-ventilated rooms.

Good artificial light is much to be preferred to insufficient sunlight. The artificial light should be sufficiently bright and steady; a fickering light is always bad. Riding against a strong wind, especially on a bicycle, may prove hurtful, at least for eyes that are inclined to any kind of inflammation. The light reflected from snow is a common source of injury to the eyes. It is a wise caution in passing from a dark room to avoid looking immediately at the sun, an incandescent light, the glistening snow, or other bright objects.

The eyes should never be rubbed, or the fingers thrust into them,[46] and much less when they are irritated by any foreign substance. The sooner the offending substance is removed the better.

Illustration: Fig. 137.—Lacrymal Canals, Lacrymal Sac, and Nasal ducts, opened by their Anterior Portion.

340. Effect of Alcohol upon the Eye. The earlier and slighter forms of injury done to the eye by the use of intoxicants are quite familiar: the watery condition of the eye and of the lids, and the red and bleared aspect of the organ. Both are the result of chronic inflammation, which crowds the blood into the vessels of the cornea, making them bloodshot and visible. The nerves controlling the circulation of the eye are partially paralyzed, and thus the relaxed vessels become distended.

But more serious results ensue. Long use of intoxicants produces diseases of the retina, involving in many cases marked diminution of acuteness as well as quickness of vision, and at times distorted images upon the surface of the retina. In other instances, the congestion of the optic nerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worse and worse, till total blindness may ensue.

It is beyond question that a wide comparison of cases by careful observers proves that a large fraction of those who indulge in strong drink suffer from some form of disease of the eye.

341. Effect of Tobacco upon Vision. Tobacco, in its distribution of evil effects, does not neglect the senses and especially the eye. A variety of vicious results is produced. The pungent smoke inflames the lids. The narcotic dilates the pupil, causing dimness and confusion of vision. A diseased condition occurs with severe pain in the eye followed by impaired vision.

Oculists speak impressively of the ill effects of tobacco, and especially of cigarettes, upon the eyes of the young. They mention a well-known disease, tobacco blindness, usually beginning with color-blindness, and progressing occasionally with increasing dimness of vision to entire loss of sight.[47]

342. The Sense of Hearing. The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending.

The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head; so deep that sounds cannot directly affect it. Some arrangement, therefore, is required for conducting the sounds inwards to this true organ.

Illustration: Fig. 138.—The Pinna, or Auricle.

In studying the structure of the ear, and how it is fitted to respond to sonorous vibrations, we may divide it into three parts: the sound-conducting part, known as the external ear, the middle ear, and the deeply placed nerve portion, the inner ear.

343. The External Ear. The external ear consists of an expanded portion known as the pinna or _auricle_, and of a passage, the auditory canal or _meatus_, leading inwards from it. The surface of the auricle is convoluted to collect and transmit the vibrations of air by which sound is produced the auditory canal conducts these vibrations to the tympanic membrane. Many animals move the auricle in the direction of the sound. Thus the horse pricks up its ears when it hears a noise, the better to judge of the direction of sounds.[48]

The external auditory meatus, the passage to the middle ear, is curved and is about an inch and a quarter long. Near its outer portion are a number of fine hairs slanting outwards to prevent the entrance of insects. Embedded in the deeper parts of the canal are glands which secrete the _cerumen_, or ear-wax, which keeps the canal moist, and helps to protect it against foreign bodies and insects. As the result of a cold, this wax may collect in sufficient quantities to block the passage, and to diminish to a considerable extent the power of hearing.

344. The Middle Ear. At the inner end of the outer ear passage is the tympanum, known as “the drum of the ear.” It is a thin, oval membrane, stretched at an angle across the deep end of the passage, which it completely closes. The tympanum is thus a partition between the passage of the outer ear and the cavity of the middle ear. On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum. Its bony walls are lined with mucous membrane similar to that lining the nose, mouth, and throat. On the inner wall of the tympanum are two openings, the round window, or _foramen rotundum_, and the oval window, or _foramen ovale_.

The tympanic cavity communicates with the back part of the throat, by the Eustachian tube. This tube is about one and a half inches long and lined with mucous membrane similar to that of the tympanic chamber and the throat. This passage is usually closed, but is opened in the act of swallowing. In health there is no communication between the chamber of the middle ear and the outside, except by the Eustachian tube. Thus a throat cold, with redness and swelling of the mucous membrane, is usually accompanied with some degree of deafness, because the swelling may block the lumen of the tube, and thus prevent the free passage of air to and fro.

Illustration: Fig. 139.—General View of the Organ of Hearing.

A, pinna;
B, cavity of the concha, showing the orifices of a great number of
sebaceous glands;
C, external auditory meatus;
D, membrana tympani;
F, incus;
H, malleus;
K, handle of malleus applied to the internal surface of the membrana
tympani;
L, tensor tympani muscle;
between M and K is the tympanic cavity;
N, Eustachian tube;
O, P, semicircular canals;
R, internal auditory canal;
S, large nerve given off from the facial ganglion;
T, facial and auditory nerves.

A most curious feature of the ear is the chain of tiny movable bones which stretch across the cavity of the middle ear. They connect the tympanic membrane with the labyrinth, and serve to convey the vibrations communicated to the membrane across the cavity of the tympanum to the internal ear. These bones are three in number, and from their shape are called the malleus, or _hammer_, incus, or _anvil_; and stapes, or _stirrup_.

The hammer is attached by its long handle to the inner surface of the drum of the ear. The round head is connected with the anvil by a movable joint, while the long projection of the anvil is similarly connected with the stirrup bone. The plate of the stirrup is fixed by a membrane into the oval window of the inner wall of the tympanic chamber.

These little bones are connected with each other and the tympanum by ligaments and moved by three tiny muscles. Two are attached to the hammer, and tighten and relax the drum; the other is attached to the stirrup, and prevents it from being pushed too deeply into the oval window.

Illustration: Fig. 140.—Ear-Bones. (Anterior View.)

1, malleus, or hammer;
2, incus, or anvil;
3, stapes, or stirrup.

345. The Internal Ear. This forms one of the most delicate and complex pieces of mechanism in the whole body. It is that portion of the organ which receives the impression of sound, and carries it directly to the seat of consciousness in the brain. We are then able to say that we hear.

The internal ear, or bony labyrinth, consists of three distinct parts, or variously shaped chambers, hollowed out in the temporal bone,—the vestibule, the semicircular canals, and the cochlea, or snail’s shell.

Illustration: Fig. 141.—A Cast of the External Auditory Canal. (Posterior view)

The vestibule is the common cavity with which all the other portions of the labyrinth connect. It is an oval-shaped chamber, about ⅓ of an inch in diameter, occupying the middle part of the internal ear. It is on the inner side of the oval window, which was closed, as we have seen, by the stirrup bone. From one side of this vestibule, or central hall, the three semicircular canals pass off, and from the other side, the cochlea.

The three semicircular canals, so called from their shape, are simply bony tubes about 1/20 of an inch in width, making a curve of about 1/4 of an inch in diameter. They pass out from the vestibule, and after bending around somewhat like a hoop, they return again to the vestibule. Each bony canal contains within it a membranous canal, at the end of which it is dilated to form an _ampulla_.

Experiment 157. _To vibrate the tympanic membrane and the little ear-bones._ Shut the mouth, and pinch the nose tightly. Try to force air through the nose. The air dilates the Eustachian tube, and is forced into the ear-drum. The distinct crackle, or clicking sound, is due to the movement of the ear-bones and the tympanic membrane.

The cochlea, or snail’s shell, is another chamber hollowed out in the solid bone. It is coiled on itself somewhat like a snail’s shell. There is a central pillar, around which winds a long spiral canal. One passage from the cochlea opens directly into the vestibule; the other leads to the chamber of the middle ear, and is separated from it by the little round window already described.

The cochlea contains thousands of the most minute cords, known as the fibers or _organ of Corti_.[49] Under the microscope they present the appearance of the keyboard of a piano. These fibers appear to vibrate in sympathy with the countless shades of sounds which daily penetrate the ear. From the hair-like processes on these tightly stretched fibers, auditory impulses appear to be transmitted to the brain.

The tubes and chambers of the inner ear enclose and protect a delicate membranous sac of exactly the same shape as themselves. Between the bony walls of the passages and the membranous bag inside is a thin, clear fluid, the _perilymph_. The membranous bag itself contains a similar fluid, the _endolymph_. In this fluid are found some minute crystals of lime like tiny particles of sand, called _otoliths_, or ear-stones. Every movement of the fluid itself throws these grains from side to side.

Illustration: Fig. 142.—Bony internal Ear of Right Side. (Magnified; the upper figure of the natural size.)

A, oval window (foramen ovale);
B, C, D, semicircular canals;
* represents the bulging part (ampulla) of each canal;
E, F, G cochlea, H, round window (foramen rotundum).

The auditory nerve, or nerve of hearing, passes to the inner ear, through a passage in the solid bone of the skull. Its minute filaments spread at last over the inner walls of the membranous labyrinth in two branches,—one going to the vestibule and the ampullæ at the ends of the semicircular canals, the other leading to the cochlea.

346. Mechanism of Hearing. Waves of sound reach the ear, and are directed by the concha to the external passage, at the end of which they reach the tympanic membrane. When the sound-waves beat upon this thin membrane, it is thrown into vibration, reproducing in its movements the character of the air-vibrations that have fallen upon it.

Now the vibrations of the tympanic membrane are passed along the chain of bones attached to its inner surface and reach the stirrup bone. The stirrup now performs a to-and-fro movement at the oval window, passing the auditory impulse inwards to the internal ear.

Every time the stirrup bone is pushed in and drawn out of the oval window, the watery fluid (the perilymph) in the vestibule and inner ear is set in motion more or less violently, according to the intensity of the sound. The membranous labyrinth occupies the central portion of the vestibule and the passages leading from it. When, therefore, the perilymph is shaken it communicates the impulse to the fluid (endolymph) contained in the inner membranous bag. The endolymph and the tiny grains of ear-sand now perform their part in this marvelous and complex mechanism. They are driven against the sides of the membranous bag, and so strike the ends of the nerves of hearing, which transmit the auditory impulses to the seat of sensation in the brain.

It is in the seat of sensation in the brain called the _sensorium_ that the various auditory impulses received from different parts of the inner ear are fused into one, and interpreted as sounds. It is the extent of the vibrations that determines the loudness of the sound; the number of them that determines the pitch.

Experiment 158. Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder. Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear.

Experiment 159. Hold a vibrating tuning-fork on the incisor teeth until you cannot hear it sounding. Close one or both ears, and you will hear it.

Experiment 160. Listen to a ticking watch or a tuning-fork kept vibrating electrically. Close the mouth and nostrils, and take either a deep inspiration or deep expiration, so as to alter the tension of the air in the tympanum; in both cases the sound is diminished.

Experiment 161. With a blindfolded person test his sense of the direction of sound, _e.g._, by clicking two coins together. It is very imperfect. Let a person press both auricles against the side of the head, and hold both hands vertically in front of each meatus. On a person making a sound in front, the observed person will refer it to a position behind him.

347. Practical Hints on the Care of the Ear. This very delicate and complicated organ is often neglected when skilled treatment is urgently needed, and it is often ignorantly and carelessly tampered with when it should be let alone.

Never insert into the ear canal the corners of towels, ear spoons, the ends of toothpicks, hairpins, or any other pointed instruments. It is a needless and dangerous practice, usually causing, in time, some form of inflammation. The abrasion of the skin in the canal thus produced affords a favorable soil for the growth of vegetable parasites.

Illustration: Fig. 143.—Diagram of the Middle and Internal Ear.

This, in turn, may lead to a chronic inflammation of the canal and of the tympanic membrane. Again, there is always risk that the elbow may be jogged and the instrument pushed through the drum-head. There is, of course, a natural impulse to relieve the itching of the ear. This should be done with the tips of the fingers or not at all.

The popular notion that something should be put into the ear to cure toothache is erroneous. This treatment does not cure a toothache, and may lead to an injury to the delicate parts of the ear. A piece of absorbent cotton, carefully inserted into the ear, may be worn out of doors, when the cold air causes pain, but should be removed on coming into the house.

Frequent bathing in the cold water of ponds and rivers is liable to injure both the ears and the general health. In salt-water bathing, the force of the waves striking against the ears often leads to earache, long-continued inflammation, or defective hearing; to diminish this risk, insert into the ears a small plug of absorbent cotton.

The ears are often carelessly exposed to cold water and inclement weather. Very cold water should never be used to bathe the ears and nostrils. Bathe moderately and gently in lukewarm water, using a wash-rag in preference to a sponge; dry gently and thoroughly. Children’s ears are often rudely washed, especially in the auditory canal. This is not at all necessary to cleanliness, and may result in a local inflammation.

Never shout suddenly in a person’s ear. The ear is not prepared for the shock, and deafness has occasionally resulted. A sudden explosion, the noise of a cannon, may burst the drum-head, especially if the Eustachian tube be closed at the time. During heavy cannonading, soldiers are taught to keep the mouth open to allow an equal tension of air.

Illustration: Fig. 144.—Section of Cochlea.

From A straight downwards is the direction of the central column, to which E points. B points to the projecting ridge, almost dividing the canal of the tube into an upper compartment (D), and a lower (C).]

Insects may gain entrance to the ears and occasion annoyance, pain, and fright, perhaps leading to vomiting, even to convulsions, with nervous children. A lighted lamp held at the entrance of the ear will often induce the offending insect to crawl out towards the light. A few drops of warm water, sweet oil, or molasses, dropped into the ear, will help remove the intruder.

When a discharge occurs from the ears, it is not best to plug them with cotton wads. It only keeps in what should be got rid of. Do not go to sleep with the head on a window sill or in any position, with the ears exposed to draughts of cold or damp air.

No effort should be made to remove the ear wax unless it accumulates unduly. The skin of the canal grows outward, and the extra wax and dust will be naturally carried out, if let alone. Never employ any of the many articles or “drops,” advertised to cure deafness. Neuralgic pain in the canal, usually classed as earache, may be due to decayed or improperly filled teeth.

Quinine, so generally used in its many preparations for malaria, causes a peculiar ringing or buzzing in the ears. This is a warning that it should be taken in smaller doses, or perhaps stopped for a time. In some cases quinine may produce temporary deafness.

The practice of snuffing up cold water into the nostrils is occasionally followed by an acute inflammation of the middle ear, some of the water finding its way through the Eustachian tube into this part of the organ of hearing. The nasal douche, so often advised as a home remedy for nasal catarrh, should be used only with great caution, and always in accordance with detailed directions from a physician.

348. Effect of Tobacco upon the Hearing. The sense of hearing is often injured by the use of tobacco. The irritating smoke filling all the inner cavity of the mouth and throat, readily finds its way up the Eustachian tube, dries the membrane, and irritates or inflames the delicate mechanism of the inner ear. Thus may be produced a variety of serious aural disturbances, such as unnatural noises, whistling, and roaring, followed oftentimes by a partial loss of hearing.

Hearing may be impaired by the use of alcoholic beverages. Alcohol inflames the mucous membrane of the throat, then by its nearness the lining of the Eustachian tube, and finally may injure the delicate apparatus of the internal ear.

Additional Experiments.

Experiment 162. Use a small pair of wooden compasses, or an ordinary pair of dividers with their points guarded by a small piece of cork. Apply the points of the compasses lightly and simultaneously to different parts of the body, and ascertain at what distance apart the points are felt as two. The following is the order of sensibility: tip of tongue, tip of the middle finger, palm, forehead, and back of hand.

Experiment 163. Test as in preceding experiment the skin of the arm, beginning at the shoulder and passing downwards. Observe that the sensibility is greater as one tests towards the fingers, and also in the transverse than in the long axis of the limb. In all cases compare the results obtained on both sides of the body.

Experiment 164. By means of a spray-producer, spray the back of the hand with ether, and observe how the sensibility is abolished.

Experiment 165. Touch your forehead with your forefinger; the finger appears to feel the contact, but on rubbing the forefinger rapidly over the forehead, it is the latter which is interpreted as “feeling” the finger.

Experiment 166. Generally speaking, the sensation of touch is referred
to the cutaneous surfaces. In certain cases, however, it is referred
even beyond this. Holding firmly in one hand a cane or a pencil, touch
an object therewith; the sensation is referred to the extremity of the
cane or pencil.
If, however, the cane or pencil be held loosely in one’s hand, one
experiences two sensations: one corresponding to the object
touched, and the other due to the contact of the rod with the skin.
The process of mastication affords a good example of the reference
of sensations to and beyond the periphery of the body.

Experiment 167. Prepare a strong solution of sulphate of quinine with the aid of a little sulphuric acid to dissolve it (_bitter_), a five-per-cent solution of sugar (_sweet_), a ten-per-cent solution of common salt (_saline_), and a one-per-cent solution of acetic acid (_acid_). Wipe the tongue dry, and lay on its tip a crystal of sugar. It is not tasted until it is dissolved.

Experiment 168. Apply a crystal of sugar to the tip, and another to the back of the tongue. The sweet taste is more pronounced at the tip.

Experiment 169. Repeat the process with sulphate of quinine in solution. It is scarcely tasted on the tip, but is tasted immediately on the back part of the tongue. Test where salines and acids are tasted most acutely.

Experiment 170. _To illustrate the muscular sense_. Take two equal iron or lead weights; heat one and leave the other cold. The cold weight will feel the heavier.

Experiment 171. Place a thin disk of _cold_ lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size. The person will judge the latter to be about the same weight, or lighter, than the single cold disk.

Experiment 172. Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other. Heat the smaller one to over 120° F., and it will be judged heavier than the larger cold one.

Experiment 173. _To illustrate the influence of excitation of one sense
organ on the other sense organs_. Small colored patches the shape and
color of which are not distinctly visible may become so when a
tuning-fork is kept vibrating near the ears. In other individuals the
visual impressions are diminished by the same process.
On listening to the ticking of a watch, the ticking sounds feebler
or louder on looking at a source of light through glasses of
different colors.
If the finger be placed in cold or warm water the temperature
appears to rise when a red glass is held in front of the eyes.

Experiment 174. _Formation of an inverted image on the retina_. Take a freshly removed ox-eye; dissect the sclerotic from that part of its posterior segment near the optic nerve. Roll up a piece of blackened paper in the form of a tube, black surface innermost, and place the eye in it with the cornea directed forward. Look at an object—_e.g._, a candle-flame—and observe the inverted image of the flame shining through the retina and choroid, and notice how the image moves when the candle is moved.

Experiment 175. Focus a candle-flame or other object on the ground-glass plate of an ordinary photographic camera, and observe the small inverted image.

Experiment 176. _To illustrate spherical aberration_. Make a pin-hole in a blackened piece of cardboard; look at a light placed at a greater distance than the normal distance of accommodation. One will see a radiate figure with four to eight radii. The figures obtained from opposite eyes will probably differ in shape.

Experiment 177. Hold a thin wooden rod or pencil about a foot from the eyes and look at a distant object. Note that the object appears double. Close the right eye; the left image disappears, and _vice versa_.

Experiment 178. _To show the movements of the iris_. It is an extremely beautiful experiment, and one that can easily be made. Look through a pin-hole in a card at a uniform white surface as the white shade of an ordinary reading-lamp. With the right eye look through the pin-hole, the left eye being closed. Note the size of the (slightly dull) circular visual field. Open the left eye, the field becomes brighter and smaller (contraction of pupil); close the left eye, after an appreciable time, the field (now slightly dull) is seen gradually to expand. One can thus see and observe the rate of movements of his own iris.

Illustration: Fig. 145.

Experiment 179. _To show the blind spot_. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm’s length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen.

Experiment 180. _To illustrate the duration of retinal impressions_. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time.

Illustration: Fig. 146.—Optic Disks. The disk A, having black and white sectors, when rotated rapidly gives an even gray tint as in B.

Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A (Fig. 146). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B.

Experiment 182._To illustrate imperfect visual judgments_. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C.

Illustration:

For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other.

Experiment 183. Make on a white card two squares of equal size. Across the one draw _horizontal_ lines at equal distances, and in the other make similar _vertical_ lines. Hold them at some distance. The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, _i.e._, both appear oblong.

Experiment 184. Look at the row of letters (S) and figures (8). To some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger. Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger.

S S S S S S S S 8 8 8 8 8 8 8 8

Experiment 185. _To illustrate imperfect visual judgment_. The length of a line appears to vary according to the angle and direction of certain other lines in relation to it (Fig. 147). The length of the two vertical lines is the same, yet B appears much longer than A.

Illustration: Fig. 147.—To show False Estimate of Size.

Experiment 186. In indirect vision the appreciation of direction is still more imperfect. While leaning on a large table, fix a point on the table, and then try to arrange three small pieces of colored paper in a straight line. Invariably, the papers, being at a distance from the fixation-point, and being seen by indirect vision, are arranged, not in a straight line, but in the arc of a circle with a long radius.