306 



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. 



