Gilbert light experiments for boys . Fig. 153. Trench faces Experiment No. 97. A ghost party. Mix a half teaspoonfnlof salt in three or four teaspoonfuls of alcohol in a saucer, standthe saucer on a cup on the table (to prevent burning the table), GILBERT LIGHT EXPERIMENTS 101 seat the party around the table in the dark, light the alcohol, andlook at your neighbors faces and at your own in a mirror. Doyou all look like ghosts ? You do, because the salt in the flamegives only yellow light, and since your rosy cheeks and rosy lipsabsorb this color they appear black. TRENCH FACES Our boys at the
Gilbert light experiments for boys . Fig. 153. Trench faces Experiment No. 97. A ghost party. Mix a half teaspoonfnlof salt in three or four teaspoonfuls of alcohol in a saucer, standthe saucer on a cup on the table (to prevent burning the table), GILBERT LIGHT EXPERIMENTS 101 seat the party around the table in the dark, light the alcohol, andlook at your neighbors faces and at your own in a mirror. Doyou all look like ghosts ? You do, because the salt in the flamegives only yellow light, and since your rosy cheeks and rosy lipsabsorb this color they appear black. TRENCH FACES Our boys at the front painted their faces black (Fig. 153)before they started out on night raids, because the black paintabsorbed the light and prevented their faces from being Fig. 154. SpectroscopeCourtesy of the Scientific American THE SPECTROSCOPE When substances are vaporized in a flame and the flame isviewed through a spectroscope (Fig. 154) the spectrum seen iscrossed by bright lines. Each substance has its own particularlines, and when we know these lines we can tell what substancesare in the flame. This is the basis of spectrum analysis. In thespectroscope shown here the light passes through a narrow slit,through tube A, through four prisms, and into the telescope Bin which the enlarged spectrum is seen. 102 GILBERT BOY ENGINEERING WHAT IS IN THE SUN AND STARS? When the light from the stars is viewed in the spectroscope,the spectrum is crossed by dark lines exactly corresponding tothe bright lines mentioned above. These are called the Fraun-hofer lines, after their discoverer. If, in the spectrum of lightfrom the sun, for example, we see dark lines exactly corre-sponding to the bright lines produced by iron in the spectrumon the earth, we know that
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