These relations show that the intensity of illumination is inversely proportional to the square of the distance from the source of light. An application of the law of intensity is made in using a simple (Bunsen) photometer. This consists of a card containing a spot soaked with oil or melted wax. (See Fig. 351.) The lights whose intensities are to be compared are placed upon opposite sides of the card. The card is then adjusted so that the spot appears the same on both sides. The illumination is now equal on both sides of the card and the _candle powers of the two lights are proportional to the squares of their distances from the card_. The simple device just described will give approximate results only. For accurate results more elaborate apparatus is required. =360. Measurement of the Intensity of Illumination.=--A standard candle (Art. 358) produces when lighted 1 candle power. The illumination caused by this upon a surface 1 ft. away and at right angles to the light rays is called a =foot-candle=. It is the unit of intensity of illumination. A 4-candle-power lamp, at a distance of 1 ft., produces 4 foot-candles. A 16-candle-power lamp at a distance of 2 ft. also produces 4 foot-candles--(16 ÷ 2²). The intensity of illumination required for a good light for seeing varies with the conditions. Thus, for stage and store lighting about 4 foot-candles are needed, while homes and churches may require but 1 foot-candle. Too great an intensity of illumination is as harmful as not enough. Exposed lights having an intensity of more than 5 candle power per square inch are often a cause of eye trouble. Such lights should be protected by frosted globes. A pleasing form of lighting for large halls and public buildings is the _indirect system_. In this, the lamps are hidden by reflectors which throw the light upon the ceiling from which it is diffused over the room. This form of lighting is more expensive than other systems since but a part of the light is reflected. Its cost therefore is an important factor when considering its use. =361. The Reflection of Light.=--The light reflected from the surfaces of bodies about us gives us information concerning our surroundings. A knowledge of the behavior of light undergoing reflection is not usually gained from ordinary observation. The law of reflection of light may be shown, however, by an experiment. [Illustration: FIG. 352.--_B´_ is as far back of the mirror as _B_ is in front of it.] [Illustration: CHRISTIAN HUYGENS (Popular Science Monthly) Christian Huygens (1629-1695). Dutch physicist; invented the pendulum clock (1656); developed the wave theory of light; discovered polarization of light (1690).] [Illustration: H. V. HELMHOLTZ "By Permission of the Berlin Photographic Co., New York." Hermann von Helmholtz (1821-1894) Germany. Established the doctrine of conservation of energy; made many discoveries in sound; invented the ophthalmoscope; established the physical basis of tone quality.]