the Catcse of coloured concentric Rings. 65- 



golng methods of seeing multiplied sets brings are become 

 familiar and easy. 



I have seen occasionally, not only foiif SMlS^e, Ml.^ven 

 six sets of concentric rings, from a very simple arrarigemetii 

 of glasses : they arise from reiterated internal reflections j 

 but it will not be necessary to carry this account of seeing 

 midtiplied 'Sets of rings to a grfcater length. ^^■"^^'"^' 



VII. Of the Size of the Rings. \ 



''^I'he diameter of the concentric rings depends lljioit the'ra- 

 dius of the curvature of the surfaces between which they are 

 formed. Curvatures of a short radius, cafteris paribus, give 

 smaller rings than those of a longer; hiit sit Isaac Newtoti 

 having already treated on this part of the subject at lafge, 

 it will not be necessary to enter further into it. 

 '*' I should however remark, that when two curving are con- 

 cerned, it is the application of them to each other that will 

 determine the size of the rings, so that large ones may be 

 produced from curvatures of a very short radius. A double 

 convex lens of 2^- inches focus, for instance, when it is laid 

 upon a double concave which is but little more in focal 

 length, gives rings that are larger than those from a lens of 

 t6 inches laid upon a plain slip of glass. 



YUi, Of Contact. rt^J;/ 



The size of the rings is considerably affected by pressure. 

 They grow larger when the two surfaces that form them afe 

 pressed closer together, and diminish when the pressure is 

 gradually removed. The smallest ring of a set rpay be in- 

 creased by this means to double and treble its former dia- 

 meter: but as the common or natural pressure of glasses laid 

 upon any flat or curved surface is occasioned by their weight; 

 the variations of pressure will not'be vcrf consiclerable when 

 they are left to assume their own distance or cbntact. To 

 produce that situation, however, which' 'is 'geiifeiPally^ called 

 contact, it will always be "necessary to' giVe a liftle'motidh 

 backwards and forwards to the incumbent lens q'i' glass, ac- 

 companied with some moderate pTessiTr6,'^fter'Wh'icli it nr'aV 

 beieft-io settle prop^riy -by its -oXv^'w^r^hi:^ 'O- ; n v..a£.i?ifr 



' F3 IX. Of 



