172 MEASUREMENT OF WAVES OF LIGHT. SECT. XX. 



The size of the rings increases with the obliquity of the inci- 

 dent light, the same colour requiring a greater thickness or space 

 between the glasses to produce it than when the light falls per- 

 pendicularly upon them. Now, if the apparatus be placed in 

 homogeneous instead of white light, the rings will all be of the 

 same colour 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 colour of the light. They 

 are largest in red, and decrease in magnitude with the succeeding 

 prismatic colours, being smallest in violet light. 



Since one of the glasses is plane and the other spherical, 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 colour, which in the undu- 

 latory system measures the length of the wave producing it 

 (N. 200). By actual measurement Sir Isaac Newton found that 

 the squares of the diameters of the brightest part of each ring 

 are as the odd numbers, 1, 3, 5, 7, &c. ; and that the squares of 

 the diameters 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 thick- 

 ness of the air corresponding to any one colour could be found, 

 its thickness for all the others would be known. Now, as Sir 

 Isaac Newton knew the radius of curvature of the lens, and the 

 actual breadth of the rings in parts of an inch, it was easy to 

 compute that the thickness of air at the darkest part of the first 

 ring is the g^ part of an inchj whence all the others have been 

 deduced. As these intervals determine the length of the waves 

 on the undulatory hypothesis, it appears that the length of a 

 wave of the extreme red of the solar spectrum is equal to the 

 0'0000266th part of an inch ; that the length of a wave of the 

 extreme violet is equal to the 0'0000167th part of an inch ; and, 

 as the time of a vibration of a particle of ether producing any 

 particular colour is directly as the length of a wave of that 

 colour, and inversely as the velocity of light, it follows that the 

 molecules of ether producing the extreme red of the solar spec- 

 trum perform 458 millions of millions of vibrations in a second ; 

 and that those producing the extreme violet accomplish 727 

 millions of millions of vibrations in the same time. The lengths 

 of the waves of the intermediate colours, and the number of their 

 vibrations, being intermediate between these two, white light, 



