150 Royal Institution. 



fringes being, caused by the action of flexion extending to different 

 distances over the different rays, is stated to be insufficient to account 

 for it, and also to account for the different colours in the fringes to 

 be made by white light. It is considered that the different flexibility 

 will account for the latter, but not for the different breadths of the 

 fringes, without another hypothesis, namely, the different ratio of the 

 force to the distance from the bending body, in different rays. 



16. The entire difference of flexion and refraction is shown from 

 the different breadths of the fringes, and from their formation upon any 

 possible hypothesis being shown to have nothing similar or analogous 

 in the phenomena of refraction, though the different flexibility is pre- 

 cisely similar to the different refrangibility, only applicable inversely 

 to the different rays. 



17. The relation of the doctrine of interference to the phenomena 

 of flexion is considered ; and it is shown that certain of these phe- 

 nomena are at variance with the doctrine. This is particularly ex- 

 emplified in the case of the phenomena observed where bodies acting 

 on light are not placed directly opposite to each other, but one be- 

 hind the other. 



18. The same phenomena are adduced to disprove M. Fresnel's 

 hypothesis, that the phenomena of flexion (termed by him diffraction) 

 depend entirely on the size of the aperture through which the light 

 enters. Three experiments are adduced in disproof of this ; the 

 first made on the aperture when the edges are directly opposite each 

 other ; the second, when the edges are moved to different distances 

 from each other on a line exactly parallel to the rays ; the third, 

 when the edges are moved on a line at any inclination to the rays. 

 In both the second and third experiment, the vertical distance of the 

 edges (I. e. the aperture) being the same, the breadth as well as the 

 separation of the fringes is found to vary with the distance of the 

 edges from each other horizontally, or in the direction of the rays. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



June 3, 1853. — On some of the Eruptive Phenomena of Iceland. 

 By Dr. John Tyndall, F.R.S. 



The surface of Iceland slopes gradually from the coast towards 

 the centre, where the general level is about 2000 feet above the 

 surface of the sea. On this, as a pedestal, are planted the Jokull 

 or icy mountains of the region, which extend both ways in a north- 

 easterly direction. Along this chain the active volcanoes of the island 

 are encountered, and in the same general direction the thermal springs 

 occur, thus suggesting a common origin for them and the volcanoes. 

 From the ridges and chasms which diverge from the mountains 

 mighty masses of steam are observed to issue at intervals, hissing 

 and roaring, and where the escape takes place at the mouth of a 

 cavern and the resonance of the cave lends its aid, the sound is like 

 that of thunder. Lower down in the more porous strata we have 

 smoking mud pools, where a repulsive blue-black aluminous paste 

 is boiled, rising at times into huge bladders, which on bursting scatter 

 their slimy spray to a height of fifteen or twenty feet. From the base 

 of the hills upwards extend the glaciers, and on their shoulders are 



