58 INTENSITY OF LIGHT. 



comparison is the light furnished by a certain kind of can- 

 dle. The lamp or the candle is placed upon one scale of a 

 balance, with the proper weight in the other scale. The 

 gas-burner is placed by the side of it, and the issue of gas 

 is so adjusted as to make the two lights equal, as shown 

 by the photometer placed near. Both lights are then al- 

 lowed to burn until the scale containing the lamp or can- 

 dle rises, showing that the prescribed amount of oil or of 

 spermaceti has been consumed. The gas is then shut off, 

 and the metre shows how much gas has been consumed. 

 By this means the quantity of light which the gas affords 

 per cubic foot is easily computed. 



Photometers, besides being useful in determining the 

 light-giving power of different kinds of candles and differ- 

 ent qualities of gas, have also been employed in comparing 

 the light coming from various other natural and artificial 

 sources. Those who have made these observations have 

 come to the conclusion that the light of the sun is equal 

 to that of between five and six thousand of the standard 

 candles, when placed at the distance of eighteen inches ; 

 that is, that to throw a light equal to the full blaze of the 

 sun upon a sheet of paper would require the combined 

 power of no fewer than five to six thousand candles placed 

 at the distance mentioned. Of course it would be practi- 

 cally impossible to place that number of candles so that 

 their light could be combined. The experiment only shows 

 what number would be necessary if the combination were 

 possible. 



As for the light of the moon, even when full, every one 

 knows that it is vastly inferior to that of the sun, but few 

 are aware how very much inferior it is. The experiments 

 of scientific men with photometers, and the computations 

 which they have made from their observations, vary con- 

 siderably in their results, as was to have been expected, 



