130 Astronomical and Nautical Collections. 



two systems to all the facts which have been hitherto dis- 

 covered, an impartial judge will not hesitate to admit the 

 superiority of the theory of undulations. 



In entering into the detail of the facts in question, it will 

 not be advisable to separate them from the theoretical 

 views which have led so immediately to their discovery ; and 

 it appears to be equally conducive to the purposes of in- 

 struction, and to the advancement of the science, to explain 

 the essential principles and foundations of a theory which 

 has been too long neglected and misunderstood : it will, 

 however, be proper on this occasion to omit many details of 

 calculation, for the sake of conciseness, and having reduced 

 each case to the condition of a mathematical problem, it will 

 be sufficient to take its solution for granted. 



The diffraction of light will be considered in the first place, 

 because it relates to the simplest possible case of a shadow cast 

 by an opaque body illuminated by a single radiant point ; and 

 this case will be considered somewhat diffusely, as affording 

 the best test of the comparative value of the two theories. 



II. Diffraction of Light, 



The term Diffraction is applied to those modifications 

 which light undergoes in passing near the extremities of 

 bodies. 



When we admit the rays of light by a very small aper- 

 ture into a dark room, we observe that the shadows of 

 bodies, instead of being terminated abruptly and distinctly 

 as they ought to be, if the light always passed by them in 

 right lines, are softened in their outlines, and bounded by 

 three very distinct fringes of colours, of unequal breadths, 

 the first being the widest, and the third the narrowest ; and 

 when the body intercepting the light is narrow, we see 

 fringes even in its shadow, which then appears to be di- 

 vided by darker and lighter stripes, placed at equal dis- 

 tances from each other. We may call this latter species 

 internal fringes, and the former external, 



Grimaldi is the first philosopher that observed and 

 studied these fringes with care. Newton, who investigated 

 the subject of diffraction, and even devoted to it the last 



