Cubes and bulbs are often mounted directly on the camera; while this restricts the choice of black and white lighting angles, the limitation can be overcome and satisfactory results achieved with little effort. The flash bulb's glamorous younger cousin, electronic flash, an even more versatile and convenient light source. It, too, produces a momentary burst of light. But it can do so again and again as long as electricity is supplied, it is even easier to use than bulbs; and it is more economical for those who take more than a few dozen flash black and white pictures every year. The idea behind the flash unit dates back to the beginnings of black and white photography; in 1851 that doughty pioneer, William Henry Fox Talbot, fastened a page of the London Times to a rotating wheel and produced a readable black and white image of it on one of his crude plates by means of an electrical spark from a battery of Leyden jars. Today's electronic flash is less like a simple spark than like a small bolt of lightning. Electricity from batteries or a wall outlet is built up to a high voltage (as much as 4,000 volts in some units) and stored in the unit's circuits; when the switch is pressed this high-voltage electricity jumps from one electrode to another inside a glass tube filled with a mixture of gases such as krypton and xenon. The electrical discharge forces these gases to glow briefly and brilliantly with a color approximating that of daylight. The burst of light is so short-1 /250,000 second or less in some specialized units-that it can be used to stop fast action, hence the alternate name "speedlight." Another frequently used name is "strobe," a hangover from the days when the newly developed device was first employed as a stroboscope-a light that flashes repeatedly at a controllable rate for studies of rapidly rotating machinery. The units now commonly used in black and white photography are not stroboscopic; they produce a single flash each time the camera shutter is released. A flash's short, bright burst of light is both an advantage and a disadvantage, for its illumination is difficult to control. Many black and white photographers prefer to use flood and spot lamps when circumstances permit. Their continuous light is easily adjusted so that the pattern of illumination-the brightness and location of highlights and shadows-can be manipulated to suit the subject. The equipment need not be elaborate-many bulbs have built-in reflectors to concentrate or spread the light, and inexpensive sockets to hold them are fitted with clamps that grip shelves or moldings so they can be placed where required. (In addition it may be helpful to have a tripod to keep the camera steady in one position while lights are being arranged; it is also advisable to use a lens shade to prevent the lamps' direct light from shining into the lens.) The basic techniques for deploying such lights are simple and with practice one can learn to create pleasing illumination for indoor scenes and portraits. The intensity of artificial light drops rapidly with distance. One foot away from the bulb shown here, a light meter receives 125 foot-candles of illumination; twice as tar away, at 2 feet, the light is cut to i/4, or about 32; at 3 feet to 1/9 or about 16. Thus the amount of light is inversely proportional to the square of the distance. Basic Black and white Lighting Techniques: continued