138 benham's artificial spectrum top. 



a white object is suddenly cut off, is at first green and fades out 

 through a more or less blue or slate colour. The object must not, 

 however, be so bright as to dazzle the eye, as the duration and 

 colour of the residual impression might in that case be very differ- 

 ent. Prof Liveing sees the colours of the discs best in bright 

 diffuse daylight, and sees hardly any colour when the discs are in 

 direct sunshine, but no two observers see the same appearances 

 under the same circumstances. 



The discs used in most of Prof Liveing's experiments, and 

 Messrs. Newton's top, are about six inches to a foot in diameter ; 

 but the size is unimportant, and the small sketch from which the 

 present figure was drawn, when placed on a turn-table, exhibited 

 colours equally bright to those on the larger tops.* The black 

 parts may either be painted on the top or made of pieces of dead- 

 black paper, and Prof Liveing used a kind of black paper, which 

 he finds to be a perfect absorbent of light of all colours, thus 

 effectually disposing of any questions as to whether the appear- 

 ances may be due to colours actually present in the black or white 

 parts of the disc. 



The experiments may be varied by making the top with white 

 arcs projecting into the black portion, and they will supply much 

 material for recreation on a dull winter's evening, when bright 

 gas or lamplight will answer admirably. 



* If, however, the top is too small and light, it cannot be made to spin 

 slowly enough to show the colours well, M'hile too large and heavy a top is 

 difficult to set spinning sufficiently rapidly. 



With reference to Crolls and Ball's theories of ice ages and 

 genial ages, Mr. Edward P. Culverwell has shown, on the basis of 

 calculations of the daily distribution of solar heat on difterent 

 latitudes at the present time and in the supposed glacial and genial 

 ages, that the winter temperature of Great Britain in the glacial age, 

 as dependent on sun heat, would be no lower than that from 

 Yorkshire to the Shetlands ; and similarly that, from 40'^ to 8o^of 

 north latitude, the shift of the winter isothermals would be only 

 about 4^ of latitude, a result wholly inadequate to produce an ice 

 age. The shift of isothermals in the genial age was found to be 

 much smaller. — Popular Science Mo7ithly. 



