WELLS AND LARSEN I LORETTOITE 669 



up by the potential; the first term varies inversely as the square 

 of the interval of 1, and the second term inversely as the interval 

 itself. When u = w the result reduces to that previously 

 found. 10 The interest attaching to the present form is that it 

 is applicable to the case in which the curve is a singular curve, 

 that is, to the path of a particle of light, whereas the previous 

 form was applicable only to curves that could be described by 

 electrons or material particles. 



If we consider the vector u as a singular vector, the first term 

 in P drops out by virtue of u.u = 0. We have therefore the 

 following result: 



In the (geometric) field of a particle of light that portion which 

 corresponds to Coulomb's law for the field of an ordinary par- 

 ticle vanishes identically. 



The field P vanishes entirely unless the particle of light (as- 

 sumed to be moving with the normal velocity of light in free 

 space) travels in a curved path so that c 1 is not parallel to u. 



MINERALOGY. — Lorettoite, a new mineral: Roger C. Wells 

 and Esper S. Larsen, Geological Survey. 



A specimen, furnished by Mr. Frank L. Hess of the United 

 States Geological Survey and received by him from Mr. I. N. 

 Wilconson of Loretto, Tennessee, proved on analysis to differ 

 chemically from any known mineral. A specimen labeled 

 "massicot," without a location, in the collections of the Univer- 

 sity of California, differs in its optical properties from any 

 known mineral, and a later optical examination of the Loretto 

 mineral showed the essential identity of the two. The name 

 lorettoite is proposed for the mineral, from its occurrence near 

 Loretto, Tennessee. 



Physical properties. The mineral is in flat, compact pieces, 

 up to an inch thick, and apparently occurs in thin seams. It 

 has a bladed structure and a very perfect cleavage along these 

 blades. The blades commonly extend across the specimen and 



10 Wilson and Lewis, loc. cit., p. 464. 

 1 Published with the permission of the Director of the U. S. Geological Survey. 



