of Newton's Rings. 415 



is a* x 0,66487; and is consequently about twenty-four times 

 greater than that of the rings of which we are treating. 



This shews that much care will be necessary to make the 

 rings visible. Suppose for instance that the incident light is po- 

 larized by a plate of tourmaline, or (which amounts to the same 

 thing) that the reflected light is examined by a tourmaline, with 

 its axis perpendicular to the plane of reflection. Few tourmalines 

 are so perfect as to transmit no more than one twenty-fourth 

 part of the light polarized perpendicular to their axis. If then 

 the rings are examined with one of these, the rings of which 

 we are in quest (whose center is bright) will be mixed with rings 

 produced by light polarized in the plane of reflexion (whose 

 center is black) of at least equal intensity : and their character 

 will therefore be entirely destroyed. If instead of a tourmaline 

 we use a doubly-refracting prism, with which both sets of rings 

 are exhibited, separated from each other, there will be no fear 

 of confusion of the rings, but a sheet of bright light (from the 

 rays polarized in the plane of reflection) will be spread over 

 the faint rings that we are seeking, and will effectually make 

 them invisible. 



The plan which I have successfully adopted is, to combine a 

 tourmaline and doubly-refracting prism. By means of the tour- 

 maline (with axis perpendicular to the plane of reflection) the 

 brightness of the sheet of light, which would otherwise cover 

 the rings that we have to examine, is so far diminished, that it 

 offers no serious obstacle. At the same time the other set of 

 rings is seen, and serves very well as an object of comparison 



To destroy the reflection at the upper surface of the imposed 

 lens is a matter of importance. I have used a plano-convex lens 

 of 5,8 inches focal length with an obtuse-angled prism placed 



