ON LIGHT. 321 



is called the penumbra in ordinary shadows, which arises 

 from the angular diameter of the sun.* Quite otherwise. 

 A shadow indeed is formed, but instead of a sharp and 

 sudden transition from darkness to light, it terminates in 

 three coloured fringes, following its contour, the inner 

 being the broadest and more distinctly coloured, the 

 outer extremely faint and feebly tinted. The order of 

 the colours, reckoning from the first dark fringe, is, 

 generally speaking, analogous to that of the colours of 

 thin plates proceeding outwards from the dark centre 

 rings, only degrading more rapidly, viz., blue within, and 

 yellow and red without. And that the tints originate in 

 the same way from the superposition of a series of dark 

 and bright fringes of the different prismatic colours, of 

 different breadths, is shown (as in the colours of thin 

 plates) by throwing on the lens in succession the several 

 coloured prismatic rays, when the fringes are seen in each 

 colour much more numerously and sharply defined, 

 being broadest in red light and narrowest in violet. 



(105.) If the object casting the shadow be long and 

 very narrow, as a hair or a strip of card not more than a 

 3oth of an inch broad, the phsenomena are still more 

 curious and complex. Besides the exterior coloured 

 fringes already described, others are seen within the 

 shadow, running parallel to its length, similarly disposed 

 along both its edges, and blending in the middle into a 



* The diffracted fringes may be seen very well on the borders of 

 shadows cast by the light of Venus when at its greatest brightness, 

 on a white surface, in a room with a single window, and under 

 favourable circumstances as to twilight. 



X 



