Management of Light in Illumination. 187 



which indicate directly and without any computation 

 the relative intensities of these lights. 



These two flat rods, which serve as a graduated scale 

 to the photometer, are about one inch in width and 

 near one quarter of an inch in thickness : they may be 

 folded up by means of joints, like a joint rule, and the 

 length of each of them may be about 10 or 12 feet. 

 Their first division is marked 10, and it is placed at 

 the distance of 10 inches from the middle of the field 

 of the photometer, when the apparatus is prepared for 

 making an experiment. 



The other divisions of this scale of light are de- 

 termined in such a manner that the numbers which 

 they bear, which I call degrees, are everywhere as the 

 squares of their distances from the middle of the field 

 of the photometer, where the two shadows are in con- 

 tact whose densities are compared and equalized. 



To fill the important office of a standard light with 

 which all others are compared, I have chosen a wax 

 candle of the first quality, just eight tenths of an Eng- 

 lish inch in diameter, and which when burning with a 

 clear and steady flame has been found to consume very 

 uniformly 108 grains Troy of wax per hour. 



To this standard light I have assigned the value of 

 100 degrees, and it is always placed exactly opposite to 

 that division of the scale of the photometer which is 

 marked 100. This division is of course at the dis- 

 tance of 31.62 inches from the middle of the field of 

 the instrument, that marked 10 being at the distance 

 of ten inches. 



These two rods are supported in a horizontal posi- 

 tion by means of light stands. 



As this apparatus is much more simple and much 



