Longden — Selenium Interference Rings. 55 



Aet. VI. — Selenium Interference Rings ; by A. C. Long-den. 



Some months ago, while at work on thin films deposited by 

 cathode discharge, at the request of Mr. C. C. Trowbridge of 

 Columbia University, I attempted to deposit for him a thin 

 conducting film of selenium. The attempt was not successful ; 

 because, although the selenium was in the conducting form in 

 the cathode, the film was always in the amorphous form, which 

 is not an electrical conductor. A number of films were depos- 

 ited under all sorts of conditions, but they were invariably 

 amorphous and therefore non-conducting. 



During the deposition of selenium, although no conducting 

 films were produced, a very interesting and beautiful phe- 

 nomenon was observed. The selenium cathode was small in 

 comparison with the size of the glass plates upon which the 

 films were deposited, and therefore the distribution of selenium 

 was not uniform, the film being considerably thicker in the 

 center than at the edges. The deposited material is sufficiently 

 transparent to transmit large quantities of light through thick- 

 nesses of several wave-lengths ; so that when light falls upon a 

 film of varying thickness, the beams reflected from the upper 

 and lower surfaces present interference phenomena similar to 

 Newton's rings. The interference bands which were at first 

 produced were so irregular that they could hardly be called 

 rings, because of the lack of uniformity in the upper surfaces 

 of the films : but after arranging the cathode in the form of 

 a small globule, films increasing uniformly in thickness toward 

 the center were obtained. 



These films, when viewed by reflected light, present con- 

 centric systems of circular interference fringes of great beauty 

 and remarkable brilliancy. The transparent selenium film is 

 deposited upon plane glass, and, having a convex upper sur- 

 face, it constitutes a very thin plano-convex lens. The inter- 

 fering beams of light, however, are not reflected from the 

 surfaces of an air film, between the convex lens and a second 

 plane, but from the two surfaces of the selenium lens itself. 

 As this lens is thickest in the center, instead of being thinnest 

 in the center as in Newton's air film, the selenium rings 

 increase instead of decrease in width and brilliancy, counting 

 from the center outward. For the same reason the order of 

 the colors is red, yellow, green, blue, violet; instead of violet, 

 blue, green, yellow, red ; and, instead of having a black center 

 when viewed by reflected light, as is the case with Newton's 

 rings, the color of the center depends upon the order of the 

 ring, counting from the margin inward. 



