Alternating Electric Arc between a Ball and Point. 123 



of fluor and Iceland spar is far less absorbent than the direct- 

 vision prisms ordinarily made with dense flint glass of a strong 

 yellow tinge. For the purpose of achromatizing lenses the 

 presence of a greenish or purplish tinge is of less importance, 

 as a smaller thickness is used : and the purple fluorescence, 

 which is objectionable in some ways, will not trouble the 

 performance of the lens for any but photographic service. 



It is of importance to note that the tint, whether purplish 

 or greenish, of fluor-spar may be greatly reduced or nearly 

 removed by simply heating the spar to a temperature a little 

 above that at which it emits light in a darkened room. Care 

 is necessary, however, that the crystals should not be heated 

 either so suddenly or to so high a temperature as to cause 

 decrepitation. Another point of even greater importance in 

 the optical applications of fluor-spar is the selection of crystals 

 wmich are not macled. Macled crystals of fluor seldom fail 

 to show, when cut, some trace of double refraction which 

 would unfit them for optical purposes. 



The refractive index of the pale green variety seems to be 

 a little higher than that of the colourless spar. I find 1'435 

 as the sodium-light index of refraction, as against 1*4338 

 found by Dr. Abbe. 



City and Guilds Technical College, 

 Finsbury, September 3, 1890. 



XV. On the Alternating Electric Arc between a Ball and 

 Point. By Edward L. Nichols*. 



Paet I.f 



THE phenomenon which forms the subject of this paper 

 w T as first brought to my notice by Mr. E. G. Acheson, 

 the result of whose unpublished observation may be briefly 

 stated as follows : — 



Two wires, w T hich formed the terminals of the secondary 

 coil of an alternating-current transformer, were brought nearly 

 into contact. One wire was armed with a ball, the other with 

 a point. When the distance was such as to admit of a dis- 

 charge between the two, it was found that a galvanometer in 

 shunt around the ball and point indicated a considerable flow 

 of continuous current. 



This phenomenon has recently been subjected to investiga- 



* From Silliman's American Journal of Science, January 1891. 

 t From experiments made bv Messrs. W. K. Archboid and G. L. 

 Teeple. 



