SIX LECTURES ON LIGHT/ 



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thousandth of an inch. But the two con- 

 ceptions are, in point of intellectual quality, 

 identical. 



N Imagine, then, a particle entering the film 

 of air where it possesses this precise thick- 

 ness. To enter the film, its attracted end 

 must be presented. Within the film it is able 

 to turn once completely round ; at the other 

 side of the film its attracted pole will be again 

 presented ; it will, therefore, enter the glass 

 at the opposite side of the film and be lost to 

 the eye. All round the place of contact, 

 wherever the film possesses this precise thick- 

 ness, the light will equally disappear we 

 shall have a ring of darkness. 



And now observe how well this conception 

 falls in with the law of proportionality dis- 

 covered by Newton. When the thickness of 

 the film is 2 </, the particle has time to per- 

 form two complete somersaults within the 

 film ; when the thickness is 3 d, three com- 

 plete somersaults ; when 10 d, ten complete 

 somersaults are performed. It is manifest 

 that in each of these cases, on arriving at the 

 second surface of the film, the attracted pole 

 of the particle will be presented. It will, 

 therefore, be transmitted, and, because no 

 light is sent to the eye, we shall have a ring 

 of darkness at each of these placss. 



The bright rings follow immediately from 

 the same conception. They occur between 

 the dark rings, the thicknesses to which they 

 correspond being also intermediate between 

 those of the dark ones. Take the case of the 

 first bright ring. The thickness of the film 

 is l /t d; in this interval the rotating particle 

 can perform only half a rotation. When, 

 therefore, it reaches t.'.e second surface of the 

 film, its repelled pole is presented ; it is, 

 therefore, driven back and reaches the eye. 

 At all distances round the centre correspond- 

 r~.g to this thickness the same effect is pro- 

 duced, and the consequence is a ring of 

 brightness. The other bright rings are sim- 

 ilarly accounted for. At the second one, 

 where the thickness is i^ d, a rotation and 

 a half is performed ; at the third, two rota- 

 tions and a half ; and at each of these places 

 the particles present their repelled poles to 

 the lower surface of the film. They are there- 

 fore sent back to the eye, producing the im- 

 pression of brightness. Here, then, we havo 

 unravelled the most subtle application that 

 Newton ever made of the Emission Theory. 



It has been stated in the early part of this 

 lecture, that the Emission Theory assigned a 

 greater velocity to light in glass and water, 

 than in air or stellar space. Here it was at 

 direct issue with the theory of undulation, 

 v/hich makes the velocity in air or stellar 

 space less than in glass or water. By an ex- 

 periment proposed by Arago, and executed 

 with consummate skill by Foucault and 

 Fizeau, this question was b -ought to a crucial 

 test, and decided in favor of the theory of 

 i.mdulation. In the present instance also the 

 two theories are at variance. Newton as- 



sumed that the action which produces the al- 

 ternate bright and dark rings took place at a 

 single surface ; that is, the second surface of 

 the film. The undulatory theory affirms 

 that the rings are caused by the interference 

 of waves reflected from both surfaces. This 

 also has been demonstrated by experiment. 

 By proper devices we may abolish reflection 

 from one of the surfaces of the film, ancj 

 when this is done the rings vanish altogether. 



Rings of feeble intensity are also formed by 

 transmitted light. These are referred by the, 

 undulatcry theory to the interference of 

 waves which have passed directly through the 

 film, with others which have suffered two re- 

 flections within the film. They are thus com- 

 pletely accounted for. 



Newton, by the foregoing exceedingly 

 subtle assumption, vaulted over the difficulty 

 presented by the colors of thin plates. And, 

 as further difficulties in process of time thick- 

 ened round the theory, his disciples tried to 

 sustain it with an ingenuity worthy of their 

 master. The new difficulties were not an- 

 ticipated by the theory, but were met by new 

 assumptions, until at length the Emission 

 Theory became what a distinguished writer 

 calls a " mob of hypotheses." In the pres- 

 ence of the phenomena of interference, the 

 theory finally broke down, while the whole of 

 these phenomena lie, as it were, latent in the 

 theory of undulation. Newton's " fits," for 

 example, are immediately translatable into 

 the lengths of the ether-waves. We have 

 the observed periodic recurrence as the thick- 

 ness varies so as to produce a retardation of 

 an odd or even number of semi-undulations.* 



Numerous other colors are due to interfer- 

 ence. Fine scratches drawn upon glass ot 

 polished metal reflect the waves of light from 

 their sides; and some, being reflected from 

 opposite sides of the same furrow, interfere 

 with and quench each other. But the ob- 

 liquity of reflection which extinguishes the 

 shorter waves does not extinguish the longer 

 ones, hence the phenomena of color. These 

 are called the colors of striated surfaces. 

 They are well illustrated by mother-of-pearl. 

 This shell is composed of exceedingly thin 

 layers, which, when cut across by the polish- 

 ing of the shell, expose their edges and fur- 

 nish the necessary small and regular grooves. 

 The most conclusive proof that the colors ar- 

 due to the mechanical state of tr.e surface i -. 

 to be found in the fact, established by Brew 

 ter, that, by stamping; the shell carefully 



* In the explanation of Newton's rings, something 

 besides thickness is to be taken into account. In the 

 case of the first surface of the film of air, the waves 

 pass from a denser to a r?rer medium, while in the 

 case of the second surface, the waves pass from a 

 rarer to a denser medium. This difference at the 

 two reflecting surfaces can be proved to be equivalent 

 to the addition of Jialf a wave-length to the thick- 

 ness of the film. To the absolute thickness, as de- 

 ermined by Newton, half a wave-length is in each 

 :ase to be added. When this is done, the dark and 

 Bright rings follow each other in exact accordance 

 with the law of interference already enunciated. 



