General Science Part 10

CHAPTER XII

PHOTOGRAPHY

120. 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 const.i.tuents, silver and chlorine.

121. The Magic Wand in Photography. Suppose we coat one side of a gla.s.s 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 pa.s.s 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 gla.s.s, 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.

[Ill.u.s.tration: FIG. 82.--A camera.]

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 pa.s.ses through the lens, falls upon the screen, and forms an image there. If we subst.i.tute 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.

Gla.s.s 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.

[Ill.u.s.tration: FIG. 83.--A negative.]

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 pa.s.sage of light, and but little light will pa.s.s through that part of the negative to the photographic paper, and consequently but little of the silver salt on the paper will be changed. On the other hand, the light portion of the negative will allow free and easy pa.s.sage of the light rays, which will fall upon the photographic paper and will change much more of the silver. Thus it is that dark places in the negative produce light places in the positive or real photograph (Fig. 84), and that light places in the negative produce dark places in the positive; all intermediate grades are likewise represented with their proper gradations of intensity.

[Ill.u.s.tration: FIG. 84.--A positive or true photograph.]

If properly treated, a negative remains good for years, and will serve for an indefinite number of positives or true photographs.

125. Light and Disease. The far-reaching effect which light has upon some inanimate objects, such as photographic films and clothes, leads us to inquire into the relation which exists between light and living things. We know from daily observation that plants must have light in order to thrive and grow. A healthy plant brought into a dark room soon loses its vigor and freshness, and becomes yellow and drooping.

Plants do not all agree as to the amount of light they require, for some, like the violet and the arbutus, grow best in moderate light, while others, like the willows, need the strong, full beams of the sun. But nearly all common plants, whatever they are, sicken and die if deprived of sunlight for a long time. This is likewise true in the animal world. During long transportation, animals are sometimes necessarily confined in dark cars, with the result that many deaths occur, even though the car is well aired and ventilated and the food supply good. Light and fresh air put color into pale cheeks, just as light and air transform sickly, yellowish plants into hardy green ones. Plenty of fresh air, light, and pure water are the watchwords against disease.

[Ill.u.s.tration: FIG. 85--Stems and leaves of oxalis growing toward the light.]

In addition to the plants and animals which we see, there are many strange unseen ones floating in the atmosphere around us, lying in the dust of corner and closet, growing in the water we drink, and thronging decayed vegetable and animal matter. Everyone knows that mildew and vermin do damage in the home and in the field, but very few understand that, in addition to these visible enemies of man, there are swarms of invisible plants and animals some of which do far more damage, both directly and indirectly, than the seen and familiar enemies. All such very small plants and animals are known as _microorganisms_.

Not all microorganisms are harmful; some are our friends and are as helpful to us as are cultivated plants and domesticated animals. Among the most important of the microorganisms are bacteria, which include among their number both friend and foe. In the household, bacteria are a fruitful source of trouble, but some of them are distinctly friends.

The delicate flavor of b.u.t.ter and the sharp but pleasing taste of cheese are produced by bacteria. On the other hand, bacteria are the cause of many of the most dangerous diseases, such as typhoid fever, tuberculosis, influenza, and la grippe.

By careful observation and experimentation it has been shown conclusively that sunlight rapidly kills bacteria, and that it is only in dampness and darkness that bacteria thrive and multiply. Although sunlight is essential to the growth of most plants and animals, it r.e.t.a.r.ds and prevents the growth of bacteria. Dirt and dust exposed to the sunlight lose their living bacteria, while in damp cellars and dark corners the bacteria thrive, increasing steadily in number. For this reason our houses should be kept light and airy; blinds should be raised, even if carpets do fade; it is better that carpets and furniture should fade than that disease-producing bacteria should find a permanent abode within our dwellings. Kitchens and pantries in particular should be thoroughly lighted. Bedclothes, rugs, and clothing should be exposed to the sunlight as frequently as possible; there is no better safeguard against bacterial disease than light. In a sick room sunlight is especially valuable, because it not only kills bacteria, but keeps the air dry, and new bacteria cannot get a start in a dry atmosphere.

CHAPTER XIII

COLOR

126. The Rainbow. One of the most beautiful and well-known phenomena in nature is the rainbow, and from time immemorial it has been considered Jehovah's signal to mankind that the storm is over and that the sunshine will remain. Practically everyone knows that a rainbow can be seen only when the sun's rays shine upon a mist of tiny drops of water. It is these tiny drops which by their refraction and their scattering of light produce the rainbow in the heavens.

The exquisite tints of the rainbow can be seen if we look at an object through a prism or chandelier crystal, and a very simple experiment enables us to produce on the wall of a room the exact colors of the rainbow in all their beauty.

[Ill.u.s.tration: FIG. 86.--White light is a mixture of lights of rainbow colors.]

127. How to produce Rainbow Colors. _The Spectrum._ If a beam of sunlight is admitted into a dark room through a narrow opening in the shade, and is allowed to fall upon a prism, as shown in Figure 86, a beautiful band of colors will appear on the opposite wall of the room.

The ray of light which entered the room as ordinary sunlight has not only been refracted and bent from its straight path, but it has been spread out into a band of colors similar to those of the rainbow.

Whenever light pa.s.ses through a prism or lens, it is dispersed or separated into all the colors which it contains, and a band of colors produced in this way is called a spectrum. If we examine such a spectrum we find the following colors in order, each color imperceptibly fading into the next: violet, indigo, blue, green, yellow, orange, red.

128. Sunlight or White Light. White light or sunlight can be dispersed or separated into the primary colors or rainbow hues, as shown in the preceding Section. What seems even more wonderful is that these spectral colors can be recombined so as to make white light.

If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed behind _A_ in a reversed position, it will undo the dispersion of _A_, bending upward the seven different beams in such a way that they emerge together and produce a white spot on the screen. Thus we see, from two simple experiments, that all the colors of the rainbow may be obtained from white light, and that these colors may be in turn recombined to produce white light.

[Ill.u.s.tration: FIG. 87.--Rainbow colors recombined to form white light.]

White light is not a simple light, but is composed of all the colors which appear in the rainbow.

129. Color. If a piece of red gla.s.s is held in the path of the colored beam of light formed as in Section 127, all the colors on the wall will disappear except the red, and instead of a beautiful spectrum of all colors there will be seen the red color alone. The red gla.s.s does not allow the pa.s.sage through it of any light except red light; all other colors are absorbed by the red gla.s.s and do not reach the eye. Only the red ray pa.s.ses through the red gla.s.s, reaches the eye, and produces a sensation of color.

If a piece of blue gla.s.s is subst.i.tuted for the red gla.s.s, the blue band remains on the wall, while all the other colors disappear. If both blue and red pieces of gla.s.s are held in the path of the beam, so that the light must pa.s.s through first one and then the other, the entire spectrum disappears and no color remains. The blue gla.s.s absorbs the various rays with the exception of the blue ones, and the red gla.s.s will not allow these blue rays to pa.s.s through it; hence no light is allowed pa.s.sage to the eye.

An emerald looks green because it freely transmits green, but absorbs the other colors of which ordinary daylight is composed. A diamond appears white because it allows the pa.s.sage through it of all the various rays; this is likewise true of water and window panes.

Stained-gla.s.s windows owe their charm and beauty to the presence in the gla.s.s of various dyes and pigments which absorb in different amounts some colors from white light and transmit others. These pigments or dyes are added to the gla.s.s while it is in the molten state, and the beauty of a stained-gla.s.s window depends largely upon the richness and the delicacy of the pigments used.

130. Reflected Light. _Opaque Objects._ In Section 106 we learned that most objects are visible to us because of the light diffusely reflected from them. A white object, such as a sheet of paper, a whitewashed fence, or a table cloth, absorbs little of the light which falls upon it, but reflects nearly all, thus producing the sensation of white. A red carpet absorbs the light rays incident upon it except the red rays, and these it reflects to the eye.

Any substance or object which reflects none of the rays which fall upon it, but absorbs all, appears black; no rays reach the eye, and there is an absence of any color sensation. Coal and tar and soot are good ill.u.s.trations of objects which absorb all the light which falls upon them.

131. How and Why Colors Change. _Matching Colors._ Most women prefer to shop in the morning and early afternoon when the sunlight illuminates shops and factories, and when gas and electricity do not throw their spell over colors. Practically all people know that ribbons and ties, tr.i.m.m.i.n.gs and dresses, frequently look different at night from what they do in the daytime. It is not safe to match colors by artificial light; cloth which looks red by night may be almost purple by day. Indeed, the color of an object depends upon the color of the light which falls upon it. Strange sights are seen on the Fourth of July when variously colored fireworks are blazing. The child with a white blouse appears first red, then blue, then green, according as his powders burn red, blue, or green. The face of the child changes from its normal healthy hue to a brilliant red and then to ghastly shades.

Suppose, for example, that a white hat is held at the red end of the spectrum or in any red light. The characteristics of white objects is their ability to reflect _all_ the various rays that fall upon them.

Here, however, the only light which falls upon the white hat is red light, hence the only light which the hat has to reflect is red light and the hat consequently appears red. Similarly, if a white hat is placed in a blue light, it will reflect all the light which falls upon it, namely, blue light, and will appear blue. If a red hat is held in a red light, it is seen in its proper color. If a red hat is held in a blue light, it appears black; it cannot reflect any of the blue light because that is all absorbed and there is no red light to reflect.