In general, the light-user has taken advantage of the decrease by increasing the amount of light used and the period during which it is used. In this manner the greatly diminished cost of light has been a marked sociological and economic influence. After Murdock made his first installation of gas-lighting in an industrial plant early in the nineteenth century, he published a comparison of the expense of operation with that of candle-lighting. He arrived at the costs of light equivalent to 1000 candle-hours as follows: 1000 candle-hours Gas-lighting at a rate of two hours per day $1.95 " " " " " three " " " 1.40 Candle-lighting 6.50 It is seen that the longer hours of burning reduce the cost of gas-lighting by reducing the percentage of overhead charges. There are no such factors in lighting by candles because the whole "installation" is consumed. This is an early example of which an authentic record is available. At the present time a certain amount of light obtained for $1.00 with efficient tungsten filament lamps, costs $2.00 if obtained from kerosene flames and about $50.00 if obtained by burning candles. In order to obtain the cost of an equivalent amount of light throughout the past century a great many factors must be considered. Obviously, the results obtained by various persons will differ owing to the unavoidable factor of judgment; however, the following list of approximate values will at least indicate the trend of the price of light throughout the century or more of rapid developments in light-production. A fair average of the retail values of fuels and of electrical energy and an average luminous efficiency of the light-sources involved have been used in making the computations. The figures apply particularly to this country. TABLE SHOWING THE APPROXIMATE TOTAL COST OF 1000 CANDLE-HOURS FOR VARIOUS PERIODS Per 1000 candle-hours 1800 to 1850, sperm-oil $2.40 tallow candle 5.00 1850 to 1865, kerosene 1.65 tallow candle 6.85 1865 to 1875, kerosene .75 tallow candle 6.25 gas, open-flame .90 1875 to 1885, kerosene .25 gas, open-flame .60 1885 to 1895, kerosene .15 gas, open-flame .40 1895 to 1915, gas mantle .07 carbon filament .38 metallized filament .28 tungsten filament (vacuum) .12 tungsten filament (gas-filled) .07