104 



Mr. W. Crookes on 



[Dec. 5, 



film of mica. The fly is supported by a hard steel cup instead of a 

 glass cup, and the needle point on which it works is connected 

 by means of a wire with a platinum terminal sealed into the glass. 

 At the top of the radiometer bulb a second terminal is sealed in. 

 The radiometer can therefore be connected with an induction coil, 

 the movable fly being made the negative pole. 



Passing over the phenomena observed at low exhaustions, the author 

 finds that, when connected with the coil, a halo of a velvety violet 

 light forms on the metallic side of the vanes, the mica side remaining 

 dark throughout these experiments. As the pressure diminishes, a 

 dark space is seen to separate the violet halo from the metal. At a 

 pressure of half a millim. this dark space extends to the glass, and 

 positive rotation commences. 



On continuing the exhaustion, the dark space further widens out 

 and appears to flatten itself against the glass, and the rotation becomes 

 very rapid. 



When aluminium cups are used for the vanes instead of disks 

 backed with mica, similar appearances are seen. The velvety violet 

 halo forms over each side of the cup. On increasing the exhaustion 

 the dark space widens out, retaining almost exactly the shape of 

 the cup. The bright margin of the dark space becomes concentrated 

 at the concave side of the cup to a luminous focus, and widens out 

 at the convex side. On further exhaustion, the dark space on the 

 convex side touches the glass, when positive rotation commences, 

 becoming very rapid as the dark space further increases in size, and 

 ultimately flattening against the glass. 



Convergence of Molecular Rays to a Focus. 



The subject next investigated is the convergence of the lines of force 

 to a focus, as observed with the aluminium cup. As this could not be 

 accomplished during rapid rotation, an instrument was made, having 

 the cup-shaped negative pole fixed, instead of movable. On exhaus- 

 tion, the convergence of the lines of force to a focus at the concave side 

 was well observed. When the dark space is very much larger than 

 the cup, it forms an irregular ellipsoid drawn in towards the focal 

 point. Inside the luminous boundary a focus of dark violet light can 

 be seen converging, and, as the rays diverge on the other side of the 

 focus, spreading beyond the margin of the dark space ; the whole 

 appearance being strikingly similar to the rays of the sun reflected 

 from a concave mirror through a foggy atmosphere. 



Green Phosphorescent Light of Molecular Impact. 



At very high exhaustions the dark space becomes so large that it 

 fills the tube. Careful scrutiny still shows the presence of the dark 

 violet focus ; and the part of the glass on which fall the rays diverging 



