24 Prof. Airy on the Phenomena oj 'Newton's Rings,asfonned 



seen at a nearly perpendicular incidence. As the latter are by 

 no means vivid, we must expect the former to be faint. 



The intensity of the rings which would be produced at the 

 same angle of incidence by light polarized in the plane of 



reflexion, found in the same way, (putting e' — s ? n y""' and 



" V & sin(i + iO 



0/ = sm(< — « )\ j g fl9 x . 664 , 87 . an( j Js consequently about 

 * nh{S+i*)J . 



twenty-four times greater than that of the rings of which we 

 are treating. 



This shows that much care will be necessary to make the 

 rings visible. Suppose for instance that the incident light is 

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

 same thing) that the reflected light is examined by a tourma- 

 line, with its axis perpendicular to the plane of reflexion. 

 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 centre is bright) 

 will be mixed with rings produced by light polarized in the 

 plane of reflexion (whose centre is black) of at least equal in- 

 tensity: and their character will therefore be entirely de- 

 stroyed. 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 reflexion) 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 

 tourmaline (with axis perpendicular to the plane of reflexion) 

 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 com- 

 parison. 



To destroy the reflexion at the upper surface of the im- 

 posed 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 upon its plane side, the obtuse angle being over 

 the centre of the lens. A drop of water was placed between 

 them. Though its refractive index differs sensibly from that 

 of the glass, yet the reflexion at the common surface of the 

 prism and lens is almost totally destroyed, for the following 

 reason. The surface of the lens is I suppose very slightly con- 

 vex, and when the drop of water is interposed, and the air- 



