198 



On the Volcanic Eruption in Sunda Strait. [Jan. 10, 



the paper already referred to ; both the mirrors stood the test 

 satisfactorily, the polish of the steel being very slightly the best. 



These experiments appear to show that the generally received 

 formulas for metallic reflection are approximately correct, but that the 

 actual intensity of the reflected light is always less than the theoretical 

 intensity, and that therefore, unless this be due to defects in the 

 metallic surfaces, the formulae do not completely express the laws of 

 metallic reflection. If, as appears to be the case, a change in the 

 reflective power of a plate can occur without any change in the values 

 of the principal incidence and azimuth, it is necessary to regard the 

 formulae as only approximately true, and there is additional reason for 

 thinking that, as Professor Stokes has suggested, three constants are 

 required to define a metal optically. 



I hope hereafter to determine the amount of light reflected by films 

 of silver chemically deposited on glass, and also to make some 

 determinations of the amount of radiant energy reflected by metallic 

 surfaces at various angles, the experiments of MM. de la Prevostaye 

 and Desains on this point having been but few in number. 



III. " F -tracts from a Report on the Volcanic Eruption in 

 Sunda Strait by Commander the Honourable F. C. P. 

 Vereker, H.M.S. ' Magpie,' dated Singapore, October 22, 

 1883." Communicated by Sir Frederick Evans, K.C.B., 

 F.R.S. Received December 15, 1883. 



[Plates 2, 3.] 



* * * On the 18th instant I entered Sunda Strait, passing east of 

 Thwart-way Island. This island had been reported to be split by the 

 eruption into several portions. This is incorrect. 



The island is intersected by low valleys in several places, these 

 being covered with, tall trees did not show so prominently formerly 

 as they do now. The whole of the vegetation having been swept 

 away by the tidal wave the island at a short distance off is apparently 

 divided, the low necks joining the higher portions being only visible 

 on close approach. 



The surface of the Strait in this neighbourhood is covered with 

 extensive fields of floating pumice stone, often in one to two foot 

 cubes, through which the ship easily forced her way. 



****** 



I inclose sketches which I trust will convey the general appearance 

 better than a written description. The whole of the neighbourhood 

 is covered with greenish-yellow mud, and all traces of vegetation 

 are everywhere destroyed. 



