(J3 REPORT ON PHYSICAL OPTICS. - 



on the coloured bands by the application of heat to the glasses, the 

 colours retreating to the edges of the plates, and the bands dimi- 

 nishing in breadth as the temperature was increased. The same 

 author also found that no sensible change took place in the phe- 

 nomenon when the air was withdrawn by the air-pump. 



In the observations of Du Tour, the reflected and the transmit- 

 ted tints were observed at the same time, the latter being reflected 

 from the second surface of the lower glass, and returning to the 

 eye through the entire system. This latter set of rings is rendered 

 more distinct, when the shadow of an opaque body is passed over 

 the upper surface. In this manner the phenomenon was observed 

 by Sir William Herschel ; and it was found that additional 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 re- 

 flector ; 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 coloured 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 Light.f 



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

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

 Iceland spar, whose principal section was parallel or perpendicular 

 to the plane of incidence, this philosopher observed that the inten- 

 sity of the light in one of the images varied with the incidence, 

 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 trans- 

 mitted, 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 reflected 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. 



* "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'Jrmeil, torn. iii. 



