132 Astronomical and Nautical Collections. 



Let us now calculate how much the breadth of the abso- 

 lute geometrical shadow must be reduced, on account of the 

 dimensions of the hole : since we have supposed its dia- 

 meter -2~}-u of an inch, the extreme rays will be at the dis- 

 tance of 3- J^ of an inch from the central raj^s, and the dis- 

 tance of the card from the wire being twice as great as that 

 of the hole, the breadth of the penumbra will be ^|^, and 

 the true shadow instead of ^\ will be 2% — -of-^, or .15 —.008 

 = .142; and this space ought to be perfectly dark if the 

 rays underwent no inflection. Now if we observe the 

 shadow attentively, we shall discover in it stripes slightly 

 illuminated, separating some dark lines, the middle line 

 being always bright : the term bright being always applied 

 to a stripe lying between two darker stripes, whatever the ac- 

 tual intensity of its light may be. This experiment, so easily 

 made, proves therefore that light is actually inflected into 

 the shadows of bodies, as Grimaldi had remarked. It is in- 

 deed very rapidly weakened as the angle of inflection is in- 

 creased ; but this rapid decrease has nothing in it unfavour- 

 able to the theory of undulations, which explains it easily 

 from the minute breadth of the separate undulations, and 

 even assigns the law of its gradation. It is, therefore, ob- 

 vious, that Newton was mistaken in denying the diflTusion 

 of light into the dark space behind an illuminated object, 

 and the objection to the theory of undulations, which he de- 

 rived from this observation, was founded on error. 



In speaking of these internal fringes, it becomes necessary 

 to describe the ingenious experiment made on these fringes 

 by Dr. Young, and the important inference which he drew 

 from them. 



Having intercepted, by the interposition of a screen, all 

 the light which passed on one side of the illuminated body, 

 he found that the fringes situated within the shadow com- 

 pletely disappeared without leaving any traces of their 

 existence, though only one half of the light concerned in 

 the experiment was withdrawn. Hence he inferred, that 

 the meeting of two beams of light was required for their 

 production, and they were the result of the influences exerted 

 by one of the two beams on the other : for since each of 



