1C6 NEWTON'S RINGS. SECT. XX. 



half illuminated, which is not found to be the case. M. 

 Fresnel, whose opinion is of the first authority, thought 

 this test conclusive. It may therefore be concluded that 

 the rings arise entirely from the interference of the 

 rays : the light reflected from each of the surfaces in 

 apparent contact reaches the eye by paths of different 

 lengths, and produces colored and dark rings alternately, 

 according as the reflected waves coincide or destroy one 

 another. The breadths of the rings are unequal ; they 

 decrease in width, and the colors become more crowded, 

 as they recede from the center. Colored rings are also 

 produced by transmitting light through the same ap- 

 paratus ; but the colors are less vivid, and are comple- 

 mentary to those reflected, consequently the central spot 

 is white. 



The size of the rings increases with the obliquity of 

 the incident light ; the same color requiring a greater 

 thickness or space between the glasses to produce it than 

 when the light falls perpendicularly upon them. Now 

 if the apparatus be placed in homogeneous instead of 

 white light, the rings will all be of the same color with 

 that of the light employed. That is to say, if the light 

 be red, the rings will be red divided by black intervals. 

 The size of the rings varies with the color of the light. 

 They are largest in red, and decrease in magnitude with 

 the succeeding prismatic colors, being smallest in violet 

 light. 



Since one of the glasses is plane and the other spheri- 

 cal, it is evident that from the point of contact, the space 

 between them gradually increases in thickness all round, 

 so that a certain thickness of air corresponds to each 

 color, which in the undulatory system measures the length 

 of the wave producing it (N. 195). By actual measure- 

 ment, Sir Isaac Newton found that the squares of the di- 

 ameters of the brightest part of each ring are as the odd 

 numbers, 1, 3, 5, 7, &c. ; and that the squares of. the diam- 

 eters of the darkest parts are as the even numbers, 0, 2, 4, 

 6, &c. Consequently the intervals between the glasses 

 at these points are in the same proportion. If, then, 

 the thickness of the air corresponding to any one color 

 could be found, its thickness for all the others would be 

 known. Now as Sir Isaac Newton knew the radius of 



