340 FOURTH REPORT — 1834. 



ditional sets of rings became visible by increasing the number of 

 reflecting faces. Sir William Herschel observed, likewise, that 

 the primary reflected system was produced when a lens was laid 

 upon a metallic reflector ; and he remarks, that in this case the 

 transmitted system must be conceived to be absorbed by the 

 metal. The same author has described a remarkable set of co- 

 loured bands adjacent to the iris, at the limit of total reflexion, 

 when a prism is in contact with a plane surface*. The analysis 

 of this phenomenon has been given by Sir John Herschel in his 

 Essay on Lightf. 



The important observations of M. Arago are the next to de- 

 mand our notice I . Viewing the rings through a rhomboid of 

 Iceland spar, whose principal section was parallel or perpendicu- 

 lar to the plane of incidence, this philosopher observed that the 

 intensity of the light in one of the images varied with the inci- 

 dence, and that it vanished altogether when the rays made an 

 angle of 35° with the surface. It was further observed, that 

 the same image vanished, and at the same angle, whether the 

 rings were formed by reflexion or transmission. Thus, the light 

 of the transmitted, as well as of the reflected rings, was wholly 

 polarized in the plane of incidence, and at the usual angle for 

 glass. M. Arago has further shown, that the colours of the re- 

 flected and transmitted rings are not only complementary, but 

 that their intensities are also precisely the same ; for, when the 

 two systems are superposed, they completely neutralize each 

 other. 



But the most remarkable of the residts obtained by this author 

 relate to the rings formed by the plate of air inclosed between a 

 lens of glass and a metallic reflector. When these were ob- 

 served in the manner already alluded to, one of the images 

 vanished, as before, at the polarizing angle of glass ; while its 

 appearance, at angles above and below the polarizing angle, pre- 

 sented a remarkable contrast. When the incidence was less 

 than this angle, the two images seen through the double refract- 

 ing crystal difi^ered only in intensity ; the dimensions and colours 

 of the rings wei-e the same in both. Beyond the polarizing 

 angle, however, the rings in the two images were of cottipletnen- 

 tary colours ; so that if the series in one commenced from a 

 black centre, in the other it began from a white one. The di- 

 mensions of the rings of the same order in the two images were 

 also different. Similar phenomena wei'e produced when the thin 



* " Experiments for investigating the Cause of the coloured Rings," &c., Phil. 

 Trans. 1807, 1809, 1810. 

 t Articles 641, 642. 

 J " Sur les Couleurs des Lames minces," Memoires d'Arriteil, tom. iii. 



