82 INTERFERENCE OF LIGHT. 



with it were known to Grrimaldi. The latter observer had 

 even explicitly asserted that " an illuminated body may be 

 rendered darker by the addition of light" and adduced a sim- 

 ple experiment in proof of it. Grimaldi's experiment was as 

 follows. Let the Sun's light be admitted into a darkened 

 chamber through two small and equal apertures of a circular 

 form. Two diverging cones of light will be thus produced ; 

 and each of these cones will be surrounded by a penumbra in 

 which the illumination is only partial. Now let these two 

 beams be received on a screen at some distance, where the 

 penumbras of the two cones overlap. It will be then ob- 

 served, that although the greater part of this doubly illu- 

 minated space is brighter than the penumbra of one cone alone, 

 yet the boundaries of the overlapping portions are much 

 darker than the other parts of the penumbras which do not 

 overlap ; and if one of the beams be intercepted by an ob- 

 stacle, this dark part will recover the brightness of the rest. 

 Thus darkness may be produced by adding light ; and, on the 

 other hand, by withdrawing a portion of the light we may 

 augment the illumination. 



(102) This interesting experiment assumed a more distinct 

 and decisive character in the hands of Young. If the two 

 apertures be reduced to a very small size, and brought close 

 together, and if the original light be homogeneous, we shall 

 observe a series of alternate bright and dark bands, formed at 

 those points where the waves proceeding from the two origins 

 conspire, or are opposed. That these alternations of light 

 and darkness are caused by the mutual action of the two 

 beams, is proved by the fact, that if one of the beams be in- 

 tercepted, the whole system of bands will disappear, and the 

 light which remains become of uniform intensity. By with- 

 drawing one of the lights, then, the dark intervals recover 

 their brightness ; so that darkness, in this case, must have 

 been produced by the action of one light on the other. 



In this and other experiments of interference, the two 



