[Illustration: FIG. 218.--Section of a lamp base and socket.] Fig. 118 shows the use of an adapter that is sometimes encountered in old electric fixtures, the use of which requires explanation. Mention has already been made of the various forms of lamp sockets in use before the Edison base became a standard. In order to use an Edison lamp in a socket intended for another form of base an adapter must be employed to suit the new base to the old socket. In the figure the piece _P₁_, is the adapter. This is intended to adapt the standard lamp base to a socket that was formerly in use on the Thompson-Houston system of electric lighting. The adapter is joined to the old socket by the screw at _G_ and the circuit formed as already described. =Lamp Labels.=--For many years all incandescent lamps were rated in candlepower and were made in sizes 8, 16, 32, etc., candlepower. On the label was printed the voltage at which the lamp was intended to operate, and also the candlepower it was supposed to develop. Thus 110 v., 16 cp. indicated that when used on 110-volt circuit, the lamp would give 16 candlepower of light. This label in no way indicated the amount of energy expended. With the development of the more efficient filaments came a tendency to label lamps in the amount of energy consumed. This has resulted in all lamps being labeled to show the voltage of the circuit suited to the lamp, and the watts of electricity consumed when working at that voltage. At present a lamp label may be marked 110 v., 40 w., which indicates that it is intended to develop its best performance at 110 volts and will consume 40 watts at that voltage. Commercial lamps are now manufactured in sizes of 10, 15, 25, 40, 60, 75, and 100 watts capacity for ordinary use. Of these the 40-watt lamp probably fulfills the greatest number of conditions and is most commonly used. Besides these there are the high-efficiency lamps of the gas-filled variety that are made in larger sizes and the miniature lamps in great variety. All are labeled to show the volts and the watts consumed. =Illumination.=--The development of high-efficiency lamps has caused a radical change in the methods of illumination. With cheaper light came the desire to more nearly approximate the effect of daylight in illumination. This has brought into use indirect illumination, in which the light from the lamp is diffused by reflection from the ceiling and walls of the room. Illuminating engineering is now a business that has to do with placing of lamps to the greatest advantage in lighting any desired space. In large and complicated schemes of lighting professional services are necessary, but in household lighting the required number of lamps for the various apartments are almost self-evident. The lighting of large rooms, however, requires thoughtful consideration and in many cases the only definite solution of the problem is that of calculation.