This stated, let us ascertain the net cost of the unit of light in each system of lighting. We shall take as a basis the Paris prices, which are generally higher than those of other countries, owing to taxes, and shall confine our researches to the eight following systems: Electricity (incandescent and arc lamps), gas (butterfly, Auer and Denayrouse burners), lamp oil, kerosene and acetylene. 1. Oil Lamp.--This method of lighting has become more and more neglected because it is the most troublesome. The mean price of the kilo is 1.6 francs. As the carcel hour consumes 42 grammes, it consequently amounts to 0.06, say 6 centimes. 2. The Incandescent Lamp.--In the scale of prices one of the oldest processes of lighting is closely followed by one of the most recent--the incandescent lamp. We shall base our calculations upon the Edison 16 candle electric lamp, which is the one most widely used. In this it takes 35 watts to obtain a carcel. As the hectowatt, the mean price of which is 15 centimes, gives approximately 3 carcels, the price of the carcel will, consequently, be 5 centimes. 3. Gas.--Gas, with the butterfly burner, burns from 125 to 130 liters to furnish the carcel. As the price of a cubic meter is 30 centimes, the carcel will cost 0.39, that is to say, 4 centimes. 4. Kerosene, the decline of which is perhaps beginning, costs about 0.75 centime per kilo. The consumption per carcel is nearly 40 grammes. It amounts, therefore, to 3 centimes. 5. The arc lamp is of very varied model. We shall take as a type those used for lighting the large boulevards. They are of 8 amperes and 50 volts; that is to say, of 4 hectowatts, and are presumed to give an illuminating power of 300 carcels. The carcel is consequently obtained with 13 watts and its net cost is 0.0195, or, approximately, 2 centimes. 6. Acetylene.--This new system of lighting has hardly as yet made its exit from the laboratory. So we must not be greatly astonished at the variations in the price at which it is claimed that it can be obtained on the two sides of the Atlantic. As a kilo of carbide of calcium gives 300 liters of acetylene, and as the minimum price of the carbide is 40 centimes per kilo in France, a cubic meter of the gas costs 1.35 franc. As it requires about 7.5 liters to give the carcel, the latter will consequently amount to 0.01; say 1 centime. 7. The Denayrouse Burner.--This burns nearly 300 liters of gas to produce 30 carcels, normally. As the photometric experiments are recent, let us suppose that it gives but 25 carcels. As 300 liters of gas represent an approximate expense of 10 centimes, we shall obtain the carcel at the price of 0.004, or at less than half a centime. 8. The Auer Burner.--This burns nearly 115 liters of gas to produce 5 carcels. The expense per carcel, with the cubic meter of gas at 30 centimes, is therefore 0.0069; say 0.7 of a centime.