228 



LIGHT. 



through space. In the undulatory theory, however, which is more generally 

 received, this velocity must be regarded as that with which the waves or un- 

 dulations of light are propagated through space in the same sense as waves 

 appear to move on the surface of water if a pebble be dropped in to form a 

 centre round which they are propagated. It is necessary to remember when 

 considering any system of undulations, no matter through what medium they 

 may be propagated, that the progressive motion which belongs to them is a 

 motion of form merely, and not of matter. The waves which are propagated 

 round a centre when a pebble is dropped into calm water, present an appear- 

 ance to the eye as though the water which formed the wave really moved out- 

 ward from the centre of the undulations. Such is, however, not the case. No 

 particle of the fluid has any progressive motion whatever, of which many 

 proofs may be offered. If any floating body be placed on the surface of the 

 water, it will not be carried along by the waves, and if similar waves be form- 

 ed, as they might be, by giving a peculiar motion to a sheet or cloth, they 

 would have the same appearance of progressive motion, although the parts of 

 the sheet or cloth, as is evident, would have no other motion than the up-and- 

 down motion that would form the apparent undulations. We are then to 

 remember that when light is propagated through space with the astonishing 

 velocity of two hundred thousand miles per second, there is no material sub- 

 stance which really has this progressive velocity ; it belongs merely to the 

 form of the pulsations, or undulations. The same observations, exactly, are 

 applicable 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 the 

 amplitude or breadth of these waves, just as in the case of the waves of the 

 sea Ave 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 successive wave and the hollow or crest of the suc- 

 ceeding one. 



For the solution of this refined problem in the analysis of light, we are in- 

 debted to Newton himself. To render clearly intelligible the mode in which 

 he solved it, let us imagine a flat plate of glass, such as A B, placed upon a 

 convex lens of glass, such as C D, but let it be imagined that the degree of 

 convexity is much less than that represented in the figure. 



The under surface of the flat plate will touch the vertext of tbe convexity 

 at V, and the further any point on the under surface is from V, the greater will 

 be the distance between the surfaces of the two glasses. Thus the distance 

 between them at 1 is less than at 2, and the distance at 2 is less than at 

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

 from V outward. 



If looking down on the plate A B, we consider the point V 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 distances between the surfaces of glass. 



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

 ticular color, produced by a prism, as red, for example, fall on the surface of 



