Geological Society. 159 



mirror, the needle was at once deflected 20 divisions of the scale ; 

 on placing the mirror outside the window so as to send the moon- 

 light perpendicularly through the window on the selenium, the 

 deflection of the needle was 40 divisions. The window was kept 

 closed during these experiments. 



On another evening when the moon shone very obliquely on tho 

 window, and the selenium was exposed on the inside of the 

 window directly to the moonlight, the needle was deflected 100 

 divisions of the scale, and the deflection increased to 150 divi- 

 sions after exposure for about 3 minutes. 



The change in the resistance of the selenium was from 60,000 to 

 70,000 ohms. 



These experiments show that the action on the selenium is due 

 principally, if not entirely, to radiations belonging to the visible 

 part of the spectrum. Light rays of all kinds, particularly the 

 greenish yellow, produce an instantaneous effect followed by a more 

 or less gradual effect, which continues to increase during exposure 

 for several minutes. 



These facts suggest two hypotheses as possible explanations, 

 which may help as guides in further experiments, but which 

 cannot be accepted as proved without further evidence. 



(1) That the light falling on the selenium causes an electro- 

 motive force in it, which opposes a battery-current passing through 

 it, the effect being similar to the effect due to polarization in an 

 electrolyte. 



(2) That the light falling on the selenium causes a change on 

 its surface akin to the change which it produces on the surface of 

 a phosphorescent body, and that in consequence of this change the 

 electric current is enabled to pass more readily over the surface of 

 the selenium. 



GEOLOGICAL SOCIETY. 



November 3rd, 1875. — John Evans, Esq., Y.P.E.S., President, 



in the Chair. 

 The following communications were read : — 



1. "On some new Macrurous Crustacea from the Kimmeridge 

 Clay of the Sub-Wealden boring, Sussex, and from Boulogne-sur- 

 Mer." By Henry Woodward, Esq., E.B.S., E.G.S. 



2. " On a new Eossil Crab from the Tertiary of New Zealand.' 

 By Henry Woodward, Esq., E.B.S., E.G.S. 



3. " On a remarkable Eossil Orthopterous Insect from the Coal- 

 measures of Britain." By Henry Woodward, Esq., E.R.S., E.G.S. 



4. " On the discovery of a Eossil Scorpion in the English Coal- 

 measures." By Henry Woodward, Esq., E.B.S., E.G.S. 



5. " The Drift of Devon and Cornwall, its origin, Correlation 

 with that of the South-east of England, and place in the Glacial 

 Series." By Thomas Belt, Esq., E.G.S. 



The author described the general characters of the drift in the 



