s.r Oe 
COLD LIGHT—-HARVEY 217 
Most persons do not realize that this radiant luminous efficiency 
tells us nothing regarding the efficiency of a firefly as a light pro- 
ducing machine. When the most efficient incandescent lamp, a tung- 
sten nitrogen-filled Mazda, glows, coal is being burned in some power 
house. Every ton of coal represents so much energy, but of this 
energy only one-half of 1 per cent, a well-known figure, appears as 
visible light. 
To compare a luminous animal with a commercial light we must 
ask what fraction of the energy of its fuel (food) appears as light. 
No one has determined this for the firefly, and the investigation 
would present special difficulties because the firefly flashes, and flash- 
ing lights can not be measured easily. We are forced to fall back 
on luminous bacteria which emit a steady light, despite the fact that 
they are the smallest luminescent creatures. I have studied such 
a bacterium, a cylindrical rod measuring 1.1p wide? and 2.2, long, 
with a volume of 0.000,000,000,017 cubic centimeter. 
Perhaps I may be pardoned if the technical details of such an 
efficiency determination are briefly outlined. Visible light is a form 
of energy and can be evaluated in a common energy unit—the calorie. 
We must measure the light produced by a single bacterium and 
express this in calories per second. Food represents the source of 
an organism’s energy, the energy input, and when burned liberates 
a maximum amount of energy, also measured in calories. We must 
measure the food utilized by the bacterium and express the energy 
input in calories per second. Then, light emitted in calories divided 
by food oxidized in calories, gives us the over-all efficiency of a bac- 
terium. 
The light measurements themselves present no particular difficul- 
ties. We can make an emulsion of luminous bacteria in sea water, 
many billions of them, count the number of bacteria per cubic centi- 
meter, measure the amount of light emitted by 1 cubic centimeter 
measure the absorption of light by bacteria in front of others, and 
calculate the amount of light in lumens which each bacterium would 
emit in all directions, provided there were no absorption. As one 
candle emits 47 lumens, the candle power of the smallest light in the 
world is easily obtained. 
The general scheme of investigating the energy input is as follows: 
Metabolism experiments in animals show that for a liter of oxygen 
consumed a certain number of gram-calories is produced by oxidation 
of the foodstuffs. A gram of tallow oxidized by a guinea pig lib- 
erates the same amount of heat and consumes the same amount of 
oxygen during combustion to CO, and H,O, as if it had been burned 
21n—0.001 millimeter—one twenty-five-thousandth of an inch. 
* The light of the bacteria is actually measured in light units. One lumen==0.0015 watt 
or 0.00036 calorie. 
