828 



Professor Dewar 



[April 1, 



Fig. 7. 



A, vacuum cup with liquid air, into which is placed a photographic film in £ 

 small metallic slide having a hole in the centre. C, a metallic slide, holding a 

 photographic film, which is exposed at ordinary temperature. 



^ Both of these are exposed to the light from a 16 candle-power lamp D, con- 

 tamed in a box. The light is diminished or increased by the diaphragm at E. 



