On a Method of viewing Newton s Rings. 245 



minals are largely separated. The electrostatic field in the 

 neighbourhood of my apparatus is extremely powerful. Long 

 sparks can be drawn from neighbouring metallic masses, such 

 as gas-pipes ; and sparks several millimetres long can be 

 obtained by presenting the knuckles to the brick walls of the 

 room in which the apparatus is placed. 



The behaviour of air and rarefied gases to powerful electric 

 stress seems to me to be analogous to the behaviour of elastic 

 solids to mechanical stresses. The initial resistance of air 

 steadily diminishes with powerful electric stresses, and under 

 a disruptive discharge sinks to two or three ohms. This 

 phenomenon leads to a rapid change of potential, and is con- 

 ducive to the formation of the electromagnetic impulses which 

 we have reason to believe are the source of the #-rays. The 

 question, moreover, of the electrical conductivity of the yether, 

 1 believe can best be considered from the elastic-solid point 

 of view. 



Jefferson Physical Laboratory, 

 Harvard University, 

 Cambridge, U.S. 



XX. On a Method of viewing Newton s Kings. 

 By T. 0. Porter*. 



IF rays of light (here supposed parallel to each other) pass 

 through a rectangular slit A (fig. 1) and fall upon a 

 piece of plate-glass of the same thickness as the width of the 

 slit or greater, and if w T e observe the reflexion of the slit, it 

 appears thus : — 



Fig. 1. 



A 



A, 

 A 2 



A 1? A 2 being the first reflexions of A in the upper and lower 

 surfaces of the glass. If the glass plate be viewed more 

 obliquely, other reflexions, which for the present we shall 

 neglect, will appear, all of them lying below A. If a second 

 glass plate be added below the first, but separated from it by 

 an interval, two more images of A will be seen below A 1? A 2 , 

 caused by the reflexion of A in the upper and lower surfaces 

 * Communicated by the Physical Society : read April 22, 1898. 



