458 HISTORY OF SCIENCE. 



Here we have figures according with those which Newton deduced 

 from the dimensions of his "rings" (page 224), although the conditions 

 of experiment and methods of measurement are entirely different in 

 the two cases. It is evident that the values of a periodicity of some 

 kind belonging to light has really been ascertained by these measure- 

 ments. In Newton's theory it was the periodicity of \hejits of easy 

 transmission and reflection ; in the undulatory theory it is the periodi- 

 city of the waves, which is represented by the amazingly minute in- 

 tervals. About the reality of the periodicity itself it is impossible to 

 entertain any doubt, and when the known range of optical phenomena 

 had not yet included polarization by reflection, and certain very curious 

 effects discovered by Fresnel and by Arago, it might seem that the 

 undulatory theory could not claim any very superior position. Both 

 theories, it should be observed, are formed from the behaviour of sen- 

 sible masses of matter, which is transferred to the supersensuous or ideal 

 particles or molecules of light. In the emission theory these particles 

 were supposed to be projected in straight lines with the velocity of 

 light. Reflection was explained by the rebound of the light-particles, 

 according to the same law that operates when a stone rebounds after 

 striking a wall, or when the billiard-ball has the direction of its motion 

 changed by impact with the cushion. Refraction was explained by 

 the illustrious expounder of the corpuscular or emission theory by the 

 aid of the conception of universal attraction, which had so largely oc- 

 cupied his mind. Newton, in fact, extended to the inconceivably 

 minute particles of light the ideas of attractive forces which he had 

 gathered from the motion of the planets. A ray of light passing through 

 the air and falling obliquely upon the surface of water, is refracted, that 

 is, the beam of light changes its direction, because in entering the liquid 

 the atoms of the latter attract the light-particles : it can be shown by 

 mathematical analysis that this would occasion the reflation of the 

 ray. Or, to be more accurate in statement, the force which operates 

 is the excess of the attraction between the light and water- particles 

 over the attraction between the light and air-particles. A mathematical 

 consequence of these suppositions is, that the light-particles must move 

 more rapidly through the water than through the air. The undulatory 

 theory, on the other hand, requires the light to pass through water less 

 rapidly than through air. To any pair of media having the same re- 

 lation in their refractive powers as the air and water here specially 

 mentioned as examples, these considerations, of course, equally apply. 

 Here, then, was a point upon which the rival theories were distinctly at 

 issue ; and a direct determination of the relative velocities of light in 

 air and water would constitute a "crucial instance.^ This crucial ex- 

 periment was subsequently made, and the result -is decisively in favour 

 of the undulatory theory. 



Huyghens, who was the first to propound the wave theory in a de- 

 finite form, laid down certain general mathematical principles regard- 



