258 



rays are supposed to pass through the window W and fall on some feebly 

 luminous object such as a heated platinum wire or a calcium sulphide 

 screen at S. Both the platinum wire and the sulphide screen were 

 used and when viewed by the eye thrcugh ground glass at various 

 angles and positions relative to the source notliing definite was noticed. 

 The feebly luminous spot at times apparently brightened, then moved 

 around in a circle and went through a series of displacements. This 

 proved that nothing definite can be arrived at by viewing directly with 

 the eye. 



The most reliable method of recording the action of a feebly luminous 

 source is photography. With this method, direct and indirect vision is 

 eliminated, as well as the error due to the increased sensitiveness of the 

 eye after being in the dark for some time. A number of photographs 

 were taken, on Seed's regular "gilt edge"' plates, with the light from a 

 heated platinum wire, a luminous calcium sulphide screen, and a feeble 

 spark. 



The Platinum Wire. 



The platinum wire was a very thin strip rut from a piece of foil 

 .03 mm. tliick, so that in no place was the wire more than .05 mm. 

 liroad. Only one place along it was allowed to be heated and the ap- 

 proximate breadth of this place was .03 mm. The wire was heated by 

 a current approximately .9 amperes from three or five Edison-Lalande 

 batteries. In some of the latter experiments a storage battery was used. 

 The relative position of the different parts of the apparatus is shown 

 in Fig. II. 



.4-^. 



B is a cardboard box in which is placed the platinum wire. The 

 platinum wire is soldered to two copper \\-ires which are fastened to a 



