ITS UNDULATIONS. 



be as smooth as if they were propelled on the calm surface 

 of a lake. 



We are then to remember that when light is propagated 

 through space with the astonishing velocity of 200,000 miles per 

 second, there is no material substance which really has this 

 progressive velocity ; it belongs merely to the form of the pulsa- 

 tions, or undulations. The same observations, exactly, are appli- 

 cable to the transmission of the waves of sound through the air. 



In order to submit the phenomena of light to a strict physical 

 analysis, it is not enough to measure the motion of its waves. 

 We require also to know their amplitude or breadth, just as, in 

 the case of the waves of the sea, we should require to know 

 not only the rate at which they are propagated over the surface of 

 the water, but also the space which intervenes between the hollow 

 or crest of each and the hollow or crest of the succeeding one. 



For the solution of this problem we are indebted to Newton 

 himself. To render intelligible the mode in which he solved it, 

 let us imagine a flat plate of glass, such as D E (fig. 2), placed 

 upon a convex lens of glass, the surface of which is represented 

 by A E, but which must be supposed to have infinitely less 

 curvature than that which appears in the figure. 



The under surface of the flat plate will touch the vertex ot 

 the convexity at c, and the further any point on the under 

 surface is from c, the greater will be the distance between the 

 surfaces of the two glasses. Thus the distance between them at 

 a is less than at c, and the distance at c is less than at <?, and so 

 on. The distance at the surfaces gradually increasing, in fact, 

 from c outward. 



If, looking down on the plate D E, we consider the point c as 

 a centre, and a circle be described round it, at all points of that 

 circle the surfaces of the glasses will have the same distances 

 between them, and the greater that circle is, the greater will be 

 the distance between the surfaces of glass. 



Having the glasses thus arranged, Newton let a beam of light 

 of some particular colour, produced by a prism, as red, for 

 example, fall on the surface of the glass D E. He found that the 

 effect produced was that a black spot appeared at the centre c, 

 where the glasses touched ; that immediately around this spot 

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



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