LIGHT. 



appeared a dark ring ; that outside of that dark ring there was 

 another circle of red light, still having the point c as its centre. 

 Outside this second circle appeared another dark ring, beyond 

 which there was another circle of red light, and so on, a series 

 of circles of red light, alternated with dark rings being formed, 

 all having the point c as their common centre. 



The distances between the surfaces of glass at which the suc- 

 cessive circles of red light were found, were too minute to be 

 directly measured, but they were easily calculated by measuring 

 the diameters of the circles of light ; and, knowing the diameters 

 of the convex surface ACS, this was a simple problem in 

 geometry, easily solved, and admitting the greatest accuracy. 



On making these calculations, Newton found that the distance 

 between the glass surfaces where the second red circle was 

 formed was double the distance corresponding to the first ; that 

 at the third red circle the distance was triple that of the first, and 

 so on. It followed, of course, that wherever the dark rings were 

 formed, the distances between the glass surfaces were not an exact 

 number of times the space corresponding to the first red circle. 



Thus if we express the space between the glasses at the first 

 red circle by 1, the space between them within that circle, 

 toward the centre c, would be a fraction. The space corre- 

 sponding to the first dark ring outside the first red circle, would 

 be expressed by 1 and a fraction ; the space at the second red 

 circle would be expressed by 2 ; the space at the second dark 

 ring would be expressed by 2 and a fraction, and so on. 



Newton was not slow to see that these phenomena were the 

 direct manifestation of those effects which, in the corpuscular 

 theory, whose nomenclature he used, corresponded to the 

 amplitude of the waves of light in the undulatory theory. The 

 space between the surfaces of glass at the first red ring was the 

 amplitude of a single wave, the space at the second red circle 

 the amplitude of two waves, and so on. Within the first red 

 circle, the space between the glasses being less than the ampli- 

 tude of a wave, the propagation of the undulation was stopped, 

 and darkness ensued ; in like manner, in the space corresponding 

 to the second dark ring, the distance between the glasses being 

 greater than the amplitude of one wave, but less than the ampli- 

 tude of two, the propagation was again stopped, and darkness 

 produced. But at the second red circle, the space being equal 

 to the amplitude of two waves, the undulations were reflected 

 and the red ring produced, and so on. 



It was evident, then, that to measure the amplitude of the 

 luminous waves, it was only necessary to calculate the distance 

 between the glasses at the first red ring. 

 202 



