84 R. T. Beatty on The Perception of the Invisible. 



acropljuies, of enemy guns, of men moving at night. The lecturer 

 next showed some remarkable experiments. 



A method was shown of navigating a model ship so as to 

 follow the course of a cable laid along the bottom of the sea. In 

 this way a ship could safely make harbour in dai-kness or fog. 

 The transmission of sound signals under water up to a distance 

 of 50 miles was described, and its application explained to 

 avoiding collisions at sea. 



Other experiments demonstrated the presence of au invisible 

 beam of light both by the fluorescent efTects produced and by the 

 production of a loud note in a telephone. The effect of inter- 

 rupting the beam by the passage of a material body across it was 

 strikingly shown. 



Morse signals from such a beam could also be recorded on a 

 moving band of paper. 



A model of a lighthouse was exhil>ited in which the light 

 was automatically switched on when darkness fell, and was 

 extinguished by the rays of the rising sun. 



Ijandscape photographs were shown, taken from an aero- 

 plane, through clouds so thick as to prevent visual observations. 



In conclusion, the lecturer remarked that work was going 

 on unceasingly to increase safety in navigation of sea and air. 

 The old enemies, darkness and fog, would always be with us, but 

 their terrors were being rapidly diminished by scientific effort. 



