DARKNESS FROM TWO LIGHTS. 197 



one light will increase the intensity of the other, 

 and the eye will see twice as much light as when 

 it received only one of the beams separately. 

 All this is nothing more than what might be 

 expected from our ordinary experience. But if 

 the difference in the distances of the two lumin- 

 ous points is only one-half of the 258th thou- 

 sandth part of an inch, or 1J, 2^, 3^, 4, times 

 that distance, the one light will extinguish the other 

 and produce absolute darkness. If the two lumin- 

 ous points are so situated, that the difference of 

 their distances from the point of the retina is 

 intermediate between 1 and 1|, or 2 and 2^, 

 above the 258th thousandth part of an inch, the 

 intensity of the effect which they produce will 

 vary from absolute darkness to double the in- 

 tensity of either light. At 1J, 2|, 3J times, &c., 

 the 258th thousandth of an inch, the intensity 

 of the two combined lights will be equal only to 

 one of them acting singly. If the lights, in 

 place of falling upon the retina, fall upon a sheet 

 of white paper, the very same effect will be pro- 

 duced, a black spot being produced in the one 

 case, and a bright white one in the other, and 

 intermediate degrees of brightness in inteime- 

 diate cases. If the two lights are violet, the 

 difference of distances at which the preceding 

 phenomena will be produced will be the 157th 

 thousandth part of an inch, and it will be inter- 

 mediate between the 258th and the 157th thou- 

 sandth part of an inch for the intermediate 

 colours. This curious phenomenon may be easily 

 shown to the eye, by admitting the sun's light 

 into a dark room through a small hole about the 

 40th or 50th part of an inch in diameter, and 



