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THOMAS YOUNG. 



more bright at that point, but when the two rays united 

 and arrived at that point together, all brightness dis- 

 appeared ; complete night succeeded to day. 



one case of the thin films; Young's great merit was the comprehen- 

 siveness of his principle; and in following out the investigation, he 

 proceeded at once to such a generalization as evinced that compre- 

 hensiveness and connected immediately those classes of phenomena 

 apparently so different in character, the thin films, the internal 

 bands, and the external fringes. When, as in Grimaldi's experiment 



( since called the phenomena of diffraction), a narrow slip of card was 

 placed in a very narrow beam of solar light, dark and bright stripes 

 parallel to the sides internally marked the whole shadow longitudi- 

 nally, while the external fringes appeared on the outside at each edge. 

 The general appearance of the shadow of a long narrow body with 

 parallel sides in a beam of solar light issuing from a minute hole in a 

 shutter, or, what is better, the focus of a small lens collecting the 

 rays to a point, is that of a shadow marked with longitudinal stripes 

 and externally bordered by parallel fringes or bands of light slightly 

 coloured, as seen in the annexed figure. 



