BREATHING LIGHT AND DARKNESS.
The following experiment, if performed with care, is exceedingly striking. Let S be a candle, whose light falls at an angle of 56° 45′ upon two plate glasses, A B, placed close to each other; and let the reflected rays, A C, B D, fall at the same angle upon two similar plates, C D, but so placed that the plane of reflection from the latter is at right angles to the plane of reflection from the former. An eye placed at E, and looking at the same time on the two plates, C and D, will see very faint images of the candle, S; which by a slight adjustment of the plates, may be made to disappear almost wholly, allowing the plate C to remain where it is. Change the position of D, till its inclination to the ray, B D, is diminished about 3°, or made nearly 53° 11′. The distance may be easily found by a little practice. When this is done, the image that had disappeared on looking into D will be restored, so that the spectator at E, upon looking into the two mirrors, C D, will see no light in C, because the candle has nearly disappeared, while the candle is distinctly seen in D. If, while, the spectator is looking into these two mirrors, either he or another person breathes upon them gently and quickly, the breath will revive the extinguished image in C, and will extinguish the visible image in D.[12]
[12] Explanation.—The light A, C, B, D is polarized by reflection from the plates A B, because it is incident at the polarizing angle 56° 45′ for glass. When we breathe upon the plates C D, we form upon their surface a thin film of water, whose polarizing angle is 53° 11′, so that if the polarized rays A, C, B, D fall upon the plates C, D, at an angle of 53° 11′, the candle from which they proceeded would not be visible, or they would not suffer reflection from the plates C D. At all the other angles the light would be reflected, and the candle visible. Now the plate D is placed at an angle of 53° 11′, and C at an angle of 56° 45′, so that when a film of water is breathed upon them, the light will be reflected from the latter, and none from the former; that is, the act of breathing upon the glass plates will restore the invisible, and extinguish the visible image.
TO SHOW THAT RAYS OF LIGHT DO NOT OBSTRUCT EACH OTHER.
Make a small hole in a sheet of pasteboard, A, and placing it upright before three candles, B, placed closely together, it will be found that the images of all the candle flames will be formed separately on a piece of paper, C, laid on the table to receive them. This proves that the rays of light do not obstruct each other in their progress, although all cross in passing through the hole.