56 Long den — Selenium Interference Rings. 



It will be seen by considering the geometrical conditions 

 which govern radiation from a point to a plane, that if our 

 cathode is a small enough globule, the radius of curvature of 

 the film will depend upon the distance of the cathode from the 

 glass plane upon which the film is deposited. Accordingly, 

 this distance determines the diameter of the rings, and the 

 width of any particular ring. By placing the cathode at a 

 suitable distance from the glass, it is very easy to obtain exceed- 

 ingly brilliant rings as much as 8 or 10 millimeters in width 

 and 4 or 5 centimeters in diameter. 



These rings are so brilliant that they are as easily projected 

 upon the screen as ordinary lantern slides ; and without the 

 usual precautions in regard to size and illumination demanded 

 in projecting Newton's rings with a glass lens and plate. They 

 should, however, be projected by reflected light. 



I am not aware that Newton's rings of this character, namely, 

 interference rings from the reflecting surfaces of a very thin 

 plano-convex lens, have heretofore been produced by any 

 process whatsoever. The physical conditions connected with 

 selenium rings are of a simpler character than those which 

 exist in the case of ordinary Newton's rings, and they may, 

 therefore, afford physicists new opportunities for the study of 

 this subject. 



Physical Laboratory of Columbia University, 

 April 20, 1900. 



