SIX LECTURES ON LIGHT- 



trurn from red to blue, the rings contracted ; 

 when the passage was from blue to red, the 

 rings expanded. When white light fell upon 

 the glasses, inasmuch as the colors were not 

 superposed, a series of iris-colored circles were 

 obtained. They became paler as the film be- 

 came thicker, until finally ihe colors became 

 so intimately reblended as to produce white 

 light. A magnified image of Newton's rings 

 is now before you, and, by employing in suc- 

 cession red, blue, and white light, we obtain 

 all the effects observed by Newton. 



He compared the tints thus obtained v ith 

 the tints of the sda--bubble, and he calcu- 

 lated the corresponding thickness. How he 

 did this may be thus made plain to you : 

 Suppose the' water of the ocean to be abso- 

 lutely smooth ; it would then accurately repre- 

 sent the earth's curved surface. Let a per- 

 fectly horizontal plane touch the surface at 

 any point. Knowing the earth's diameter, 

 any engineer or mathematician in this room 

 could tell you how far the sea's surface will 

 lie below this plane, at the distance of a yard, 

 ten yards, a hundred yards, or a thousand 

 yards from the point of contact of the plane 

 and the sea. It is common, indeed, in lev- 

 elling operations, to a'. low for the curvature 

 of the earth. Newton's calculation was pre- 

 cisely similar. His plane glass was a tan- 

 gent to his curved cne From its refractive 

 index and focal distance he determined the 

 diameter of the sphere of which his curved 

 glass formed a segment, he measured the 

 distances of his rings from the place of con- 

 tact, and he calculated the depth between the 

 tangent plane and the curved surface, exactly 

 as the engineer would calculate the distance 

 between his tangent plane and the surface of 

 the sea. The wonder is, that, where such 

 infinitesimal distances arc involved, Newton, 

 with the means at his disposal, could have 

 worked with such marvellous exactitude. 



To account for these rings was the great- 

 est difficulty that Newton ever encoun- 

 tered. He quite appreciated the difficulty. 

 Over his eagle-eye there was no film no 

 vagueness in his conceptions. At the very 

 outset his theory was confronted by the ques- 

 tion, "Why, when a beam of light is incident 

 on a transparent body, are some of the light- 

 particles reflected and some transmitted ? Is 

 it that there are two kinds of particles, the 

 one specially fitted for transmission and the 

 other for reflection? This cannot be the 

 reason ; for, if we allow a beam of light 

 which has been reflected from one piece of 

 glass to fall upon another, it, as a general 

 rule, is also divided into a reflected and a 

 transmitted portion. Thus the particles once 

 reflected are not always reflected, nor are the 

 particles once transmitted always transmitted. 

 Newton saw all this ; he knew he had to ex 

 plain why it is that the self-same particle is 

 at one moment reflected and at the next mo- 

 ment transmitted. It could only be through 

 change in the condition of the particle 



itself. The self-same particle, he affirmed, 

 was affected by " fits" of easy transmission 

 and reflection. 



If you are willing to follow me while I un- 

 ravel this theory of fits, the most subtle, per- 

 haps, that ever entered the human mind, the 

 intellectual discipline will repay you for the 

 necessary effort of attention. Newton was 

 chary of stating what he considered to be the 

 cause of the fits, but there cannot be a doubt 

 that his mind rested on a mechanical cause. 

 Nor can there be a doubt that, as in all 

 attempts at theorizing, he was compelled to 

 fall back upon experience for the materials of 

 his theory. His course of observation anqf 

 of thought may have been this : From a 

 magnet he might obtain the notion of at- 

 tracted and repelled poles. What more 

 natural than that he should endow his light- 

 particles with such poles ? Turning their 

 attracted poles towards a transparent sub- 

 stance, the particles would be sucked in and 

 transmitted ; turning their repelled poles, 

 they would be driven away or reflected. 

 Thus, by the ascription of poles, the trans- 

 mission and reflection of the self-same parti- 

 cle at different times might be accounted for. 

 Regard these rings of Newton as seen in 

 pure red light : they are alternately bright 

 and dark. The film of air corresponding to 

 the outermost of them is not thicker than an 

 ordinary soap-bubble, and it becomes thinner 

 on approaching the centre ; still Newton, as 

 I have said, measured the thickness corre- 

 sponding to every ring and showed the differ- 

 ence of thickness between ring and ring. 

 Now, mark the result. For the sake of con- 

 venience, let us call the thickness of the film 

 of air corresponding to the first dark ring d, 

 then Newton found the distance correspond- 

 ing to the second dark ring' 2 d; the thick 

 ness corresponding to the third dark ring: 3 d; 

 the thickness corresponding to the tenth dark 

 ring 10 d, and so on. Surely there must be 

 some hidden meaning in this little distanced, 

 which turns up so constantly ? One can im- 

 agine the intense interest with which Newton 

 pondered its meaning. Observe the probably 

 outcome of his thought. He had endowed 

 his light-particles with poles, but now he is 

 forced to introduce the no'. ion of periodic re- 

 currence. How was this to be done ? By 

 supposing the light -particles animated, not 

 only with a motion of translation, but 

 also with a motion of rotation. Newton's 

 astronomical knowledge would render all such 

 conceptions familiar to him. The earth has 

 such a motion. In the time occupied in pass- 

 ing over a million and a half of miles of its 

 orbit that is in twenty four hours our 

 planet performs a complete rotation, and, in 

 the time required to pass over the distance d, 

 Newton's light-particle must be supposed to 

 perform a complete rotation. True, the 

 light-particle is smaller than the planet and 

 the distance d, instead of being a million and 

 a half of miles, is a little over the ninety- 



