42 BLACK LINES ATTEMPT TO ACCOUNT FOR. 



The difference in the lengths of the violet light must equally 

 correspond with the fractional part of an inch, before described 

 as peculiar to that colored ray, that is, the .00000157th ; and 

 for the other, the difference must be intermediate in the relative 

 order in which they stand in the spectrum. See Dr. Thomas 

 Young ; Huygens, as the originator of the idea of the Undu- 

 latory System of Light ; and the admirable observations of 

 Mrs. Somerville. 



The phenomena described are considered favorable to Huy- 

 gens's system of undulation. I freely admit the possibility of 

 the exercise of undulation, but consider that the action of the 

 precise and increased differences in the length of homogeneous 

 rays, and in what are called heterogeneous rays, of perfect 

 light, may be accounted for otherwise. With respect to both, 

 in their extended state, on the spectrum of Newton, as afforded 

 by his triangular prism, they exhibited the striated appearance 

 observed by Wollaston, and described with such precision by 

 Fraunhoffer, and amounting to 574 black lines distributed in 

 the. 360 parts of the visible spectrum, and 26 outside, which 

 make in the entire 600. 



Is there not, probably, some agency exercised by these dark 

 lines to affect, at certain lengths of distance, vibrations capable 

 in one proportional length to be out of the way of interference ; 

 at another, to reduce the intensity of the two rays directed to 

 the one point on the paper, to appear not more than can be pro- 

 duced of intensity by one of the rays only ; and in a third case, 

 at another certain distance, to obstruct illumination, and extin- 

 guish light altogether ? 



May not that total obscurity be produced by the collision of 

 the black lines of each beam or ray, at a certain point of dis- 

 tance of each ray, so as to produce total darkness, in the same 

 manner as the shadows from two candles or lamps when cross- 

 ing each other produce intensity of davit shade ? 



With respect to the cause of the existence of the dark lines 

 which intersect the spectrum of refracted light, as produced by 



