NIGHT PHOTOGRAPHS, TAKEN BY THE LIGHT OF THE ARC LAMPS.] The arc lamp is largely used for the illumination of wide streets, public squares, railway stations, and the exteriors of theatres, music-halls, picture houses, and large shops. The intense brilliancy of the light produced may be judged from the accompanying photographs (Plate IX.), which were taken entirely by the light of the arc lamps. Still more powerful arc lamps are constructed for use in lighthouses. The illuminating power of some of these lamps is equal to that of hundreds of thousands of candles, and the light, concentrated by large reflectors, is visible at distances varying from thirty to one hundred miles. Arc lamps are also largely used for lighting interiors, such as large showrooms, factories or workshops. For this kind of lighting the dazzling glare of the outdoor lamp would be very objectionable and harmful to the eyes, so methods of indirect lighting are employed to give a soft and pleasant light. Most of the light in the arc lamp comes from the positive carbon, and for ordinary outdoor lighting this carbon is placed above the negative carbon. In lamps for interior lighting the arrangement is frequently reversed, so that the positive carbon is below. Most of the light is thus directed upwards, and if the ceiling is fairly low and of a white colour the rays are reflected by it, and a soft and evenly diffused lighting is the result. Some light comes also from the negative carbon, and those downward rays are reflected to the ceiling by a reflector placed beneath the lamp. Where the ceiling is very high or of an unsuitable colour, a sort of artificial ceiling in the shape of a large white reflector is placed above the lamp to produce the same effect. Sometimes the lamp is arranged so that part of the light is reflected to the ceiling, and part transmitted directly through a semi-transparent reflector below the lamp. The composition of the light of the arc lamp is very similar to that of sunlight, and by the use of such lamps the well-known difficulty of judging and matching colours by artificial light is greatly reduced. This fact is of great value in drapery establishments, and the arc lamp has proved a great success for lighting rooms used for night painting classes.