FARM SANITATION 611 



was adopted by the United States July 1, 1909, and is now the 

 legal unit of light, and is practically equal to the standard 

 candle. 



The art of measuring the illumination of any source of 

 light is called photometry. The principle involved consists 

 in placing the source of light, or the lamp to be measured and 

 a standard lamp whose candle power is known, at such dis- 

 tances from a screen that the intensity of the light from each 

 is equal. As the light from a lamp passes out in all directions, 

 it is to be expected that the intensity of the light at all points 

 on the surface of a sphere at a certain radius from the source 

 will be equal. As the surfaces of spheres vary as the square 

 of their radii, the intensity of light varies inversely as the 

 square of the distance from the source. This assumes that 

 the source of light is a sphere, which is not true. 



Kerosene Lamps. Kerosene lamps are still in common 

 use, and, although they have some very serious objections, 

 their merits should not be entirely overlooked. In the first 

 place kerosene lamps are cheap as far as first cost is con- 

 cerned. The fuel is cheap and can be obtained almost any- 

 where. Kerosene lamps are quite safe; in fact, they excel 

 many others in this respect. There is more danger in the 

 matches than in the lamps themselves. The lamps are 

 readily portable, which is not true of all sources of artificial 

 light. 



On the other hand there are many disadvantages. The 

 odor of kerosene lamps is not pleasant, although far more 

 offensive to some persons than to others. Kerosene lamps 

 require attention in the way of trimming the wicks and clean- 

 ing the chimneys. If a large number of lamps are to be 

 cared for, the time required daily is considerable. Much 

 heat- is developed by a kerosene light, which at times may 

 be a serious disadvantage. The lamp also consumes a large 



