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 
4 
(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. 
