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A. Lesson in Science For Children and Grown-Ups By TENNEY L. DAVIS ‘fle ancients supposed that the sky was a sort of bowl inverted over the earth.It was painted blue on the inside, fastening stirs were stuck on at night, and by day the sun was driven in his enter across,the heavens from east to west. During the night the sun went back from west to east, around the out side of the bowl in order that hie feht ir did not disturb the sleep of men—and was ready in the morning to begin his daily journey again. The Chaldeans believed that the earth was flint, an island surrounded on al sides by sea. Beyond the sea were mountains on which was supported “that inverted how we call the sky.” We know today that the sky is just space. “It isn’t a definite something with the stars stuck on. The stars are ‘way out there, moving rapidly, some near, some so far away that we cannot imag ine the distance, shining all the time either by their own light or by the ght which is reflected by them from the other heavenly bodies... And space, or at least, the pheulie corner of space which constitutes our solar system, filled with white light which comes from the sun. Yet, for all that the sky is just noth ing at all, for all that it is a mere emp tiness filled with white light, it shows a wide variety of colors. On a fair day the sky is blue--and It is always deepest blue on the side opposite the sun. After a hot day the sun sets red. Poets describe a wild rose as being of the “color of dawn, and the sunset of a fall day colors the western heaven s urple or red. Once or twice during a Summer there is ‘a peacock or turquoise sunset, of the same color as the sky but more intense. — All of these color effects are derived from the white light of space, White a Mixture of All Colors White light in a mixture of lights of all the colors, if white light falls upon a surface which reflects it perfactly, like the surface of a silver mirror, the reflected light will be white, if it falls upon an object which reflects only red light and absobs all of the other colors of the spectrum, the reflected light will be red and the object will appear to be red—and indeed the object will be red, for to be red is to be capable of re flecting red light better than light of any other color, lo i. beam. of pure red light falls upon some white object, that is, upon some object which is capable of reflecting the pure white leht which ia a mixture of all the colors, there will be only red ght to reflect and the white object will ap pear to be red. If such a beam falls upon an object which does not reflect red, the object will not have any color to reflect and will appear black. In the theatre, when the man who oper on the spotlight slips a piece of red ginas In between the are lamp and the Inna, he cute off,—at least in large ‘art—all of the Nght of the are, which is not red Meht. All Heht-colored ob jects on the stage appear red, all white objects, and all whose color is auch that they refreat any red Upht. Of the fines which the operator puta in his spotlight were really pure red, all other objecta on the stage could appear black. As a matter of fact, It [a im nowsibie fs make a pure red glass. The fuss which $8 potunly sed cuts off, in nine part, light of all colors ,except red, hen wo geoxthicght reflected from a white surface it appears red because red predominates | 80 ready. When it gtrizes an object which does not reflect red, we see the other colors and the object appears green—for green is the color which is “complimentary” to red. If we could take white light and take red away, the reauit would appear to the eye to be green lght. When the spotlight throws red light on the stage, light colored objects appear red, dark colored objects appear green. If I look out upon the world through rosy spectacles, a thing either looks rosy to me or I do not see it at all. There may be blues and greens (the colors of despondency and jealousy) in the things themselves, but these colors never reach my eyes for the reason that all ight except Hight of a pare rosy hue is intercepted and absorbed by my rosy spectacles. In short, my eyes do not receive all the Nght which they might receive—and the cynic is largely right who says that “he who looks out upon the world through rosy Holdwrcies is half ‘blind.’ Setting aside the moral aspect of the issue, the cynic’s point of view has at least the advantage of being scientifi cally correct. Air Not Blue Itself The earth is surrounded by an atmos phere which is dense near the earth and thinner and thinner at greater dis tances until at a distance of, Bay, 60 miles from the the earth’s surface It is so thin that space at that place is practically a vacuum. The atmosphere is a sort of shell around the planet, and all light which comes to us from the sun has first to penetrate this shell. But the atmosphere is also a median Which reflects. Light travels in straight ness and, if some of it were not re flected by the atmosphere, none would ever enter windows on that side of a house which is away from the sun. A moment's consideration will make it evident that the apparent blue color of the sky cannot be due to the atmos phere's being blue. If that were the case, the color of the sunset ought to be a deep blue, for at that time the suns rays are obliged to travel an es pecially thick layers of atmosphere. Sunlight coming to ua through a thick layer of atmosphere appears reddish or yellow; sunlight reflected to us from the sky,,from the atmosphere, or from apace itself, appears bluish. We there fore conclude that the atmosphere has no real color of its own... ‘ Light is a wave motion, and a beam of light of a aingts pure color consists strictly of similar waves, '.A beam of white Hecht conatats, of relatively. ion waves, of relatively short waves and 9 many other waves of wave lengths In termediate between the two..The long er waves are red, the shorter ones are blue and violet, and the others are of the Intermediate colors of the spectrum. All of these waves, even those of red light, which are longest from crest to crest, are exceedingly small.. But they are larger than some of the magnitudes with which the physicist has to deal. We know that water is made of atoms, yet under the most powerful microscope which it is possible to construct, water phene to be perfectly homogenous. The ight waves sweep around the aroma and are no more disturbed by them than any Atlantia roller is disturbed by a nat of anchored mines. Bases tette If white ght faila upon’ fi gloud ‘or p hank of for tha little droplets of water are large enough to reflect all of the wives, the song as well as the ‘short ones, the refleated Haht is white, and the ate appears to be white. A rugged oil reflects equally the Atlantic rollar and the ripple which is caused by the drop of falling rain. A pebble will not reflect them both; the ripple will be turned back but the Atlantic roller will sweep on. So,when the white light of the sun encounters in the atmosphere small floating particles of dust and dirt—salt from the ocean, dust from the volcanoes, ote—and in outer space wandering small particles of matter, cosmic dust, which is not attached to any particular planet, these particles reflect some of all the various waves which go to make up the white light; but, since some among them are small enough to let the lower red waves pass while they intercept and reflect the shorter blue waves, the light which is refleeted from the sky is white light mints some of the longer red waves—or, what is the same thing, it is white light with a preponderance of the shorter blue waves. And the sky appears to be blue. The Color of Smoke The fact that the quarter of the sky which is opposite the sun is always. the brightest blue of any part of the sky is further evidence that the blue is due to the manner in which the light is re flected by small particles. In that quarter of the heavens, the opportunity for the reflection of, the shorter waves and for the total escape of the longer Waves in greatest. But the best proof of all is to be found in the colors of the setting sun. At that time, whatever light is reflected by the small particles is wholly pre vented from reaching my eyes—for I stand behind the particles with refer ence to the sun, and whatever Iight reaches my eyes is Heht‘ which they allow to pass. In the light which they allow to pass there is an excess of the Shorter wave lengths and the sunset ap pears yellow or red. There is no truth in the common be lef that a red sun at sunset indicates that the next day will be a hot one unless it is true that one hot day in dicates that the next one will probably be hot also. There is no doubt that the sun sets red after a hot day. After a hot day there is always much finely divided material, suspended in the ats mosphere. 5 Tobacco smoke consists of gates, chiefly carbon dioxide and ammonia. Carving a large number of very small particles, largely carbon, in suspension. The experiment in which the sampler blows a mouthful of smoke through his handkerchief is merely an experiment in which the suspended malter jaultered out. The smoke gives an asplena brown stain to the handkerchief. —° But this stain is not nicotine. It is merely the collection of the small par ticles which, if they had been left in the smoke, would have given it is characteristic blueness. Tobacco smoke is blue when we look at it. It is brown or red or yellow when we look: through Smoke which comes directly. from the burning tobacco .appoirs blue; ‘after: tt haea been inhaled by the lungs: it ap pears mutch graver. ‘The lungs have removed many of the small particles from it. Just as the handkerchief did in the experiment which has been de sorbed, , Mills in white because it is a suspen sion of many globules of butter fat in an otherwise colorless and transparent so lution of crseln and mille angar, Akimmed mille is blue because the Inver globules have been removed, Both Blue and Yellow The affent.of very small particles may be ilustrated prettily by a simple ex veriment,, ° Tf some ‘tedinon gubstande, sayy shold, -Avhted! thi shale gin. wae ta clagolved ‘Inj p nal ta, alana ing it ‘ sallow. ie a itanang ni drop at a time, to water which is vice tlvaly auirwed, tha ahellie will be aipated ot in the form of Gxaeedin nay small’ particles, so small Indeed ¢ they will not ‘readily Bettie. ‘out of the aura ai.d. cannot be removed by fltra~ ion. A point will soon be reached at Which the particles of shellac are suffi ciently abundant to give to the solution a_sky-blue color to one who holds It be tween himsalf and the Heht. If the so lution is held against a dark surface so that the Heht which reaches the eye is Mght which is reflected from It, It will appear to be of a pale yellow color. The clearest blue sky ia clear blue be cause it is not altogether clear, but con tains a great many small particles. In deed, if outer space were really a per fect vacuum, It ls doubtful if our‘days would be more than twilights except In those places Which are exposed ‘to the direct' rays of the sun. Many of the affects of lght are due to Phenomena connected with reflection, and the ‘clear blue eye,” which Helms holtz has pointed out, “lg onky a turbid M medium,’ owes its very clearness to the fact that it turns back upon in some of the Heltz which reaches it. The amvall Particlea which it contains send back some of the shorter wives, while they allow some of the longer ones to pass, (Copyright, 101p, by ‘Tenney 1. Davie
Newspaper Details
Boston Globe
Boston, Massachusetts, US
Sun, Jul 06, 1919
Page 203
Full Page
Clipped by
Nicole F.
USA 31 Aug 2026
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