General Science - Bertha May Clark (children's books read aloud TXT) 📗
- Author: Bertha May Clark
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FIG. 81.—The nearsighted eye. The defect is remedied by concave glasses.
118. Headache and Eyes. Ordinarily the muscles of accommodation adjust themselves easily and quickly; if, however, they do not, frequent and severe headaches occur as a result of too great muscular effort toward accommodation. Among young people headaches are frequently caused by over-exertion of the crystalline muscles. Glasses relieve the muscles of the extra adjustment, and hence are effective in eliminating this cause of headache.
An exact balance is required between glasses, crystalline lens, and muscular activity, and only those who have studied the subject carefully are competent to treat so sensitive and necessary a part of the body as the eye. The least mistake in the curvature of the glasses, the least flaw in the type of glass (for example, the kind of glass used), means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache and general physical discomfort.
119. Eye Strain. The extra work which is thrown upon the nervous system through seeing, reading, writing, and sewing with defective eyes is recognized by all physicians as an important cause of disease. The tax made upon the nervous system by the defective eye lessens the supply of energy available for other bodily use, and the general health suffers. The health is improved when proper glasses are prescribed.
Possibly the greatest danger of eye strain is among school children, who are not experienced enough to recognize defects in sight. For this reason, many schools employ a physician who examines the pupils' eyes at regular intervals.
The following general precautions are worth observing:—
1. Rest the eyes when they hurt, and as far as possible do close work, such as writing, reading, sewing, wood carving, etc., by daylight.
2. Never read in a very bright or a very dim light.
3. If the light is near, have it shaded.
4. Do not rub the eyes with the fingers.
5. If eyes are weak, bathe them in lukewarm water in which a pinch of borax has been dissolved.
CHAPTER XII PHOTOGRAPHY120. The Magic of the Sun. Ribbons and dresses washed and hung in the sun fade; when washed and hung in the shade, they are not so apt to lose their color. Clothes are laid away in drawers and hung in closets not only for protection against dust, but also against the well-known power of light to weaken color.
Many housewives lower the window shades that the wall paper may not lose its brilliancy, that the beautiful hues of velvet, satin, and plush tapestry may not be marred by loss in brilliancy and sheen. Bright carpets and rugs are sometimes bought in preference to more delicately tinted ones, because the purchaser knows that the latter will fade quickly if used in a sunny room, and will soon acquire a dull mellow tone. The bright and gay colors and the dull and somber colors are all affected by the sun, but why one should be affected more than another we do not know. Thousands of brilliant and dainty hues catch our eye in the shop and on the street, but not one of them is absolutely permanent; some may last for years, but there is always more or less fading in time.
Sunlight causes many strange, unexplained effects. If the two substances, chlorine and hydrogen, are mixed in a dark room, nothing remarkable occurs any more than though water and milk were mixed, but if a mixture of these substances is exposed to sunlight, a violent explosion occurs and an entirely new substance is formed, a compound entirely different in character from either of its components.
By some power not understood by man, the sun is able to form new substances. In the dark, chlorine and hydrogen are simply chlorine and hydrogen; in the sunlight they combine as if by magic into a totally different substance. By the same unexplained power, the sun frequently does just the opposite work; instead of combining two substances to make one new product, the sun may separate or break down some particular substance into its various elements. For example, if the sun's rays fall upon silver chloride, a chemical action immediately begins, and as a result we have two separate substances, chlorine and silver. The sunlight separates silver chloride into its constituents, silver and chlorine.
121. The Magic Wand in Photography. Suppose we coat one side of a glass plate with silver chloride, just as we might put a coat of varnish on a chair. We must be very careful to coat the plate in the dark room,[B] otherwise the sunlight will separate the silver chloride and spoil our plan. Then lay a horseshoe on the plate for good luck, and carry the plate out into the light for a second. The light will separate the silver chloride into chlorine and silver, the latter of which will remain on the plate as a thin film. All of the plate was affected by the sun except the portion protected by the horseshoe which, because it is opaque, would not allow light to pass through and reach the plate. If now the plate is carried back to the dark room and the horseshoe is removed, one would expect to see on the plate an impression of the horseshoe, because the portion protected by the horseshoe would be covered by silver chloride and the exposed unprotected portion would be covered by metallic silver. But we are much disappointed because the plate, when examined ever so carefully, shows not the slightest change in appearance. The change is there, but the unaided eye cannot detect the change. Some chemical, the so-called "developer," must be used to bring out the hidden change and to reveal the image to our unseeing eyes. There are many different developers in use, any one of which will effect the necessary transformation. When the plate has been in the developer for a few seconds, the silver coating gradually darkens, and slowly but surely the image printed by the sun's rays appears. But we must not take this picture into the light, because the silver chloride which was protected by the horseshoe is still present, and would be strongly affected by the first glimmer of light, and, as a result, our entire plate would become similar in character and there would be no contrast to give an image of the horseshoe on the plate.
[Footnote B: That is, a room from which ordinary daylight is excluded.]
But a photograph on glass, which must be carefully shielded from the light and admired only in the dark room, would be neither pleasurable nor practical. If there were some way by which the hitherto unaffected silver chloride could be totally removed, it would be possible to take the plate into any light without fear. To accomplish this, the unchanged silver chloride is got rid of by the process technically called "fixing"; that is, by washing off the unreduced silver chloride with a solution such as sodium thiosulphite, commonly known as hypo. After a bath in the hypo the plate is cleansed in clear running water and left to dry. Such a process gives a clear and permanent picture on the plate.
122. The Camera. A camera (Fig. 82) is a light-tight box containing a movable convex lens at one end and a screen at the opposite end. Light from the object to be photographed passes through the lens, falls upon the screen, and forms an image there. If we substitute for the ordinary screen a plate or film coated with silver chloride or any other silver salt, the light which falls upon the sensitive plate and forms an image there will change the silver chloride and produce a hidden image. If the plate is then removed from the camera in the dark, and is treated as described in the preceding Section, the image becomes visible and permanent. In practice some gelatin is mixed with the silver salt, and the mixture is then poured over the plate or film in such a way that a thin, even coating is made. It is the presence of the gelatin that gives plates a yellowish hue. The sensitive plates are left to dry in dark rooms, and when the coating has become absolutely firm and dry, the plates are packed in boxes and sent forth for sale.
Glass plates are heavy and inconvenient to carry, so that celluloid films have almost entirely taken their place, at least for outdoor work.
123. Light and Shade. Let us apply the above process to a real photograph. Suppose we wish to take the photograph of a man sitting in a chair in his library. If the man wore a gray coat, a black tie, and a white collar, these details must be faithfully represented in the photograph. How can the almost innumerable lights and shades be produced on the plate?
The white collar would send through the lens the most light to the sensitive plate; hence the silver chloride on the plate would be most changed at the place where the lens formed an image of the collar. The gray coat would not send to the lens so much light as the white collar, hence the silver chloride would be less affected by the light from the coat than by that from the collar, and at the place where the lens produced an image of the coat the silver chloride would not be changed so much as where the collar image is. The light from the face would produce a still different effect, since the light from the face is stronger than the light from the gray coat, but less than that from a white collar. The face in the image would show less changed silver chloride than the collar, but more than the coat, because the face is lighter than the coat, but not so light as the collar. Finally, the silver chloride would be least affected by the dark tie. The wall paper in the background would affect the plate according to the brightness of the light which fell directly upon it and which reflected to the camera. When such a plate has been developed and fixed, as described in Section 121, we have the so-called negative (Fig. 83). The collar is very dark, the black tie and gray coat white, and the white tidy very dark.
The lighter the object, such as tidy or collar, the more salt is changed, or, in other words, the greater the portion of the silver salt that is affected, and hence the darker the stain on the plate at that particular spot. The plate shows all gradations of intensity—the tidy is dark, the black tie is light. The photograph is true as far as position, form, and expression are concerned, but the actual intensities are just reversed. How this plate can be transformed into a photograph true in every detail will be seen in the following Section.
124. The Perfect Photograph. Bright objects, such as the sky or a white waist, change much of the silver chloride, and hence appear dark on the negative. Dark objects, such as furniture or a black coat, change little of the chloride, and hence appear light on the negative. To obtain a true photograph, the negative is placed on a piece of sensitive photographic paper, or paper coated with a silver salt in the same manner as the plate and films. The combination is exposed to the light. The dark portions of the negative will act as obstructions to the passage of light, and but little light
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