238 



Prof. E. Frankland on Dry Fog. 



[Jan. 9. 



perpendicular to the plane of incidence, the effect of magnetisation is 

 to introduce a component into the reflected ray perpendicular to the 

 original plane of polarisation, whose amplitude, c, is given in the 

 several cases by the following equations, in which i is the angle of 

 incidence, and r of reflection, and h a small constant depending prin- 

 cipally on C. and the intensity of the incident ray: — 1. When the 

 magnetisation is normal to the reflecting surface. If the incident ray 

 be polarised in the plane of incidence — 



^ (1 + cos 2 /•) sin 2 i sin 2 i 



sinr . sin 2 (<" + /') . cos (<' — /') 



If it be polarised in a plane perpendicular to the plane of incidence — 



7 cos 2 r . sin 2 i sin 2i 

 C=k—. — — r- : . 



sin r . sin 2 (/-[-/') .cos (/' — /') 



2. When the magnetisation is parallel to the intersection of the sur- 

 face and the plane of incidence, and the plane 'of polarisation of the 

 incident ray is either in or perpendicular to the plane of incidence — 



7 cos r sin 2 i sin2i 



G=.k—— . 



sin a (i + rj cos (i—r) 



This vanishes at the grazing and normal incidences, and, in the case 

 of iron, attains a maximum at about the angle of incidence i = 

 63= 20'. 



I do not obtain any change of phase by reflection in any case ; and 

 this is to be expected, as this change of phase probably depends on 

 the nature of the change from one medium to another, which, follow- 

 ing M'Cullagh, I have uniformly assumed to be abrupt. Apart from 

 this question of change of phase, my results conform completely to 

 Mr. Kerr's beautiful experiments on the reflection of light from the 

 pole of a magnet, as published in the Philosophical Magazines for 

 May. 1677. and March, 1878. 



in. " On Dry Fog." By E. FRANKLAND, D.C.L., F.R.S., Pro- 

 fessor of Chemistry in the Royal School of Mines. Received 

 October 29, 1878. 



It has often been noticed, especially in and near large towns, that 

 a foggy atmosphere is not always saturated with moisture : thus on 

 the 17th of October last, at 8"30 p.m., during a thick fog in London, 

 the degree of humidity was only 80 per cent, of saturation ; and 

 Mr. Glaisher. in his memorable balloon ascents, observed that in 

 passing through cloud or fog the hygrometer sometimes showed the 

 air to possess considerable dryness. In the ascent from Wolver- 



