346 



Professor Oliver Lodge 



[June 1, 



The radiation from the sphere is still too strong, but it can be 

 stopped down by a diaphragm plate with holes in it of varying size, 

 clamped on the sending box, Fig. 21. 



Having thus reduced the excursion of the spot of light to a foot or 

 so, a metal plate is held as reflector, and at once the spot travels a 

 couple of yards. A wet cloth reflects something, but a thin glass 

 plate, if dry, reflects next to nothing, being, as is well known, too 

 thin to give anything but " the black spot." I have fancied that it 

 reflects something of the 3-in. waves. 



With reference to the reflecting power of different substances it 



Fig. 21. 



General arrangement for optical experiments; showing Metal Box on a Stool, 

 inside which the Radiators were fixed (this box was really like Fig. 20) ; 

 the Copper Hat containing the Coherer, with the metal Box containing 

 Battery and Galvanometer Coil connected to it by a compo pipe conveying 

 the wires ; a Paraffin Prism ; and a Polarising Grid. 



may be interesting to give the following numbers, showing the motion 

 of the spot of light when 8-in. waves were reflected into the copper 

 hat, the angle of incidence being about 45°, by the following 

 mirrors : — 



Sheet of window glass or at most 1 division. 



Human body 7 divisions. 



Drawing board 12 „ 



Towel soaked with tap-water 12 „ 



Tea-paper (lead?) 40 



Dutch metal paper 70 „ 



Tinfoil 80 



Sheet copper 100 and up against stops. 



