ffaar 



.MP T1I<X THEORY t>F LIGHT A N D C ItCTUTl 9, 



e^e cniinat dwciriminate them from one,) 

 this contused or duplicated pulse, whose 

 strongest part precedes, and whose weakest 

 follows, does produce on the retina, tiie 

 sensation of a yeUow. If tiiese surfticBs are 

 further removed asunder, the weaker pulse 

 may become coincident with the" reflection of 

 the " second," or next following pulse, from 

 the first surface, " and lagg behind thatalso, 

 and be coincident with the third, fourth, 

 fifth, sixth, seventh, or eighth — ; so that, if 

 there be a thin transparent body, that from 

 the greatest thinness requisite to produce co- 

 lours, does by degrees grow to the greatest 

 thickness, — the colours shall be so often re- 

 peated, as the weaker pulse does lose paces 

 with its primary or first pulse, and is coinci- 

 dent with a" sub^equeiu " pulse. And this, 

 as it is coincident, or follows from tiie first 

 hypothesis, 1 took of colours, so upon expe- 

 riment have I found it in multitudes of in- 

 stances that seem to prove it." (P. 65. . 6?.) 

 This was printed about seven years before 

 any of Newton's experiments were made. We 

 are informed by Newton, that Hooke was 

 afterwards disposed to adopt his " sugges- 

 tion" of the nature of colours ; and yet it 

 does not appear that Hooke ever applied 

 that improvement to his explanation of these 

 phenomena, or inquired into the necessary 

 consequence of a change of obliquity, upon 

 his original supposition, otherwise he could 

 not but have discovered a striking coinci- 

 dence with the measures laid down by New- 

 ton from experiment. All former attempts, 

 to explain the colours of thin plates, have 

 either proceeded on suppositions which, like 

 Newton's, would lead us to ex[)ect the 

 greatest irregularities in the direction of the 

 refracted rays ; or, like Mr. Michell's, would 



re<iwi!re skich effects frolffl tlier ehkwigp of the 

 angle of incidence, as are contrary, to the ef- 

 fects observed ; or they are equally deficient 

 with respect to both these circumstances, 

 atid are inconsistent with the most moderate 

 attention to the principal phenomena. 



Corollary hi. OftJie Colours of thick 

 Plates. 



When a beam of light passes through a re- 

 fracting surface, especially if imperfectly po- 

 lished, a portion of it is irregularly scinterod^ 

 and makes the surface visible in all directionsv 

 but most conspicuously in directions not far 

 distant from that of the light itself: and, if 

 a reflecting surface be placed parallel to the 

 refracting surface, this scattered light, as 

 well as the principal beam, will be reflected, 

 and there will also be a new dissipation of 

 light, at the return of the beam through the 

 refracting surface. These two portions of 

 scattered light will coincide in direction; 

 and, if the surfaces be of such a form as to 

 collect the similar effects, will exhibit rings of 

 colours. The interval of retardation is, here, 

 the difference between the paths of the prin- 

 cipal beam and of the scattered light be- 

 tween the two surfaces ; of course, wherever 

 the inclination of the scattered light is equal 

 to that of the beam, although in diflerent 

 planes, die interval will' vanish, and all the 

 undulations will conspire. At other inclina- 

 tions, the interval will be the differerice of 

 the secants from the secant of the inclination 

 or angle of refraction of the principal beam. 

 From these causes, all the colours of con- 

 cave mirrors, observed by Newton and others, 

 are necessary consequences : and it appears 

 that their production, though somewhat simi- 

 lar, is by no means, as Newton imagined. 



