HEItSCHEL ON COLOURED RINGS. 133 



plain glass. Nor can it be successful unless all the forego- 

 ing- methods of seeing multiplied sets of riugs are become 

 familiar and easy. 



I have seen occasionally, not only four and five, but even 5 or G sets of 

 six sets of concentric rings, from a very simple an-angement rul S s - 

 of glasses : they arise from reiterated internal reflections ; 

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

 multiplied sets of rings to a greater length. 



VII. Of the Size of the Rings. 



The diameter of the concentric rings depends upon the Sizeofthe 



radius of the curvature of the surfaces between which they nngs * 



are formed. Curvatures of a short radius, cretcris paribus, 



give smaller rings than those of a longer; but Sir I. Newton 



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



it will not be necessary to enter farther into it. 



I should however remark, that, when two curves are con- Inverselyas<the 

 . . •.• n 1 i 1 ,i -ii angle of con- 



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



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



produced from curvatures of a very short radius. A double 



convex lens of 2^-mches 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 c i6 inches 



laid upon a plain slip of glass. 



VIII. Of Contact. 



The size of the rings is considerably affected by pressure. Pressing the 



Thev srow larger when the two surfaces that form them are surfaces to S e ~ 

 J 5 e , 1 i- • • 1 1 • ther enlarges 



pressed closer together, and diminish when the pressure is the rings. 



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

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

 ter ; 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 very considerable, 

 when they are left to assume their own distance or contact. 

 To produce that situation, however, which is generally cal- 

 led contact, it will always be necessary, to give a little mo- 

 tion backwards and forwards to the incumbent lens or glass, 

 accompanied with some moderate pressure, after which it 

 may be left to settle properly by its own weight. 



IX. 



