At a given potential at the ends of a circuit the power available in the circuit equals V²/R, i.e, it is greater, the lower the resistance R. Consequently, in a circuit of 39 bulbs, whose total resistance is less than that of 40, a greater power will be available, therefore the temperature of the heated filaments will be higher and they will give more light. So the room will be lighter from the 39 bulbs than from the 40.
Of course it is not possible to reduce the number of lamps very far simply because they will burn out.
ELECTRICITY 225