DARKNESS FROM TWO LIGHTS. 259 



which the first beam fell, is the 258th thousandth part 

 of an inch, or exactly twice, thrice, four times, &c., that 

 distance; and if this second beam falls upon the same 

 point of the retina, the one light will increase the inten- 

 sity 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 luminous points is only one-half 

 of the 258th thousandth part of an inch, or 1 J, 2-J, 3^, 4^ 

 times that distance, the one light will extinguish the other, 

 and produce absolute darkness. If the two luminous points 

 are so situated, that the difference of their distances from 

 the point of the retina is intermediate between 1 and 1 J, 

 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 intensity of either 

 light. At 1J, 2J, 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 produced ; a 

 black spot being produced in the one case, and a bright 

 white one in the other, and intermediate degrees of bright- 

 ness in intermediate cases. If the two lights are violet, 

 the difference of distances at which the preceding phe- 

 nomena will be produced will be the l7th thousandth 

 part of an inch, and it will be intermediate between the 

 258th and the 157th thousandth 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 receiving the light on a 

 sheet of paper. If we hold a needle or piece of slender 

 wire in this light, and examine its shadow, we shall find 



