IKiT. 111. 



DISSERTATION SECOND. 75 



ed of green and red ; those that succeeded became gradually 

 more dilute and ended in whiteness. It was possible to count 

 as far as seven. 



The colours of these rings were so marked by peculiari- 

 ties in shade and vivacity, that Newton considered them as 

 belonging to different orders ; so that an eye accustomed 

 to examine them, on any particular colour of a natural ob- 

 ject being pointed out, would be able to determine to what 

 order in this series it belonged. 



Thus we have a system of rings or zones surrounding a 

 dark central spot, and themselves alternately dark and 

 coloured, that is, alternately transmitting the light and re- 

 flecting it. It is evident that the thickness of the plates of 

 air interposed between the glasses, at each of those rings, 

 must be a very material element in the arrangement of this 

 system. Newton, therefore, undertook to compute their 

 thickness. Having carefully measured the diameters of the 

 first six coloured rings, at the most lucid part of each, he 

 found their squares to be as the progression of odd numbers 

 1, 3, 5, 7, &c. The squares of the distances from the centre 

 of the dark spot to each of these circumferences, were, 

 therefore, in the same ratio, and consequently the thickness 

 of the plates of air, or the intervals between the glasses, 

 were as the numbers 1, 3, 5, 7, &c. 



When the diameters of the dark or pellucid rings which 

 separated the coloured rings were measured, their squares 

 were found to be as the even numbers 0, 2, 4, 6, and, there- 

 fore, the thickness of the plates through which the light was 

 wholly transmitted were as the same numbers. A great 

 many repeated measurements assured the accuracy of these 

 determinations. 



As the curvature of the convex glass on which the flat 

 surface of the plano-convex rested, was known, and as the 

 diameters of the rings were measured in inches, it was easy 



