438 ON A NEW SPECIES OF COLOURED FRINGES 



ber of reflexions, are crossed by other fringes, inclined to 

 them at a small angle. At an angle of about 44°, the image 

 formed by four reflexions, is covered with interfering fringes, 

 but it is not till the angle of incidence is greater, that this irre- 

 gularity is distinctly seen on the image formed by two re- 

 flexions. 



Hitherto I had observed no fringes upon the first or bright 

 image, which is obviously composed of light that has not suf- 

 fered reflection from the second plate of glass. By concealing^ 

 however, the bright light of the first image, so as to perceive 

 the image formed by a second reflection, within the first plate, 

 and by viewing this image through a small aperture, which I 

 found of the greatest service in giving distinctness to all the 

 phenomena, I observed fringes across the first image, far sur- 

 passing in precision of outline, and in richness of colouring, 

 every analogous phenomenon which I had seen. When these 

 fringes were concealed, I also observed other fringes on the 

 image immediately behind them, and formed by a third re- 

 flexion, from the interior of the first plate. I now concealed 

 the second image, upon which the fringes were extremely 

 bright, and very faint stripes were seen upon the one imme- 

 diately behind it. 



In examining these phenomena a little more attentively, I 

 observed that the size of the fringes in the first image, varied 

 with the distance of the eye from the plates, while those on the 

 second and fourth image diminished with that distance. 



The magnitude of this change will be understood from the 

 following experiments : 



Number of Fringes across the first Images Number of Fringes across second image. 



Angles of Inci- 

 dence. 



Eye near. 



Eye 



a few inches 

 distant. 



Eye near. 



Eye 



a few inches dis- 

 tant. 



0° 









6 





6i 



47 



*T 





** 



5 





51 



61 



3i 





3 



4 





4f 



73 



% 





2i 



3 





8f 



When 



