Note 12 — PART III. Scientific Pursuits: (continued 3)

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.