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the glass, A B. He found that the effect produced was that a black spot ap- 

 peared at the centre, V, where the glasses touched ; that immediately around 

 this spot there appeared a circle of red light ; that beyond that circle appeared 

 a dark ring; that outside of that dark ring there was another circle 

 of red light, still having the point V 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 be- 

 ing formed, all having the point V as their common centre. 



The distances between the surfaces of glass at which the successive 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 C V D, this was a simple problem 

 of 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 distance between the glass surfaces were not an exact num- 

 ber 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 V, would be a 

 fraction. The space corresponding 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 manifes- 

 tation 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 s 

 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 amplitude of a wave, the propagation of the undu- 

 lation was stopped, and darkness ensued ; in like manner, in the space corre- 

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

 than the amplitude of one wave, but less than the amplitude of two, the propa- 

 gation 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. 



When light of other colors was thrown upon the glass, a similar system of 

 luminous rings was produced, but it was found in each case that the first ring 

 varied in its diameter according to the color of the light, and consequently that 

 the amplitude, of the waves of lights of different colors are different. It ap- 

 peared that the waves of red light were the largest ; orange came next to 

 them ; then yellow, green, blue, indigo, and violet, succeeded each other, the 

 waves of each being less than those of the preceding. But the most astonish- 

 ing part of this most celebrated investigation was the minuteness of these 

 waves. It appeared that the waves of red light were so minute, that forty 

 thousand of them would be comprised within an inch, while the waves of violet 

 light, forming the other extreme of the series, were so small, that sixty thou- 

 sand spread over an inch, and the waves of light of other colors were of inter- 

 mediate magnitudes. 



