ON^ tH^B flrtl^onT OF LTGWT A'WI>C<)tO'UK9: 



02^ 



idteM1!tcul with tlife production of those 6f thirt 

 plates-. It is indeed stirprisittg', that it did ribl 

 occur to so accurate a reasoner, tliat the 

 colours of thin plates are always lost in 

 white light, after ten or twelve alternations, 

 ■while, in this case,, they are supposed-to bft 

 distinguishable itftfir utariy thousiinds. 



GonoLLARY IV. Of Colours by inflection. 

 . VVhatever may be the cause of the inflec- 

 tion of light passing through a small aper- 

 ture, the light nearest its centre must be the 

 least diverted, and the nearest to its sides 

 the most: another portion of light, falling 

 ver3^ obliquely on the margin of the aper- 

 ture, will be copiously reflected in various 

 directions; some of which will either per- 

 fectly' or very nearly coincide in direction 

 with the unreflecled light, and having taken 

 a circuitous route, will so interfere with it, as 

 to cause an appearance of colours. The 

 length of the two tracks will differ the less, 

 as the direction of the reflected light has been 

 less changed by its reflection, that is, in the 

 light passing nearest' to tl>e margin; so that 

 the blues will appear in the light nearest the 

 ihadow. The effect will be increased and 

 modified, when the reflected light falls 

 within the influence of the opposite edge, so 

 as to interfere with the light simply inflected 

 by that also. 



On the supposition that inflection is pro- 

 duced b}' the effect of an ethereal atmosphere, 

 varying as a given power of the distance from 

 a centre, I have constructed a diagram, (Plate 

 14. Fig. 108.) with the assistance of calcula- 

 tions similar to those by which the effect of at- 

 mospherical refraction is determined, show- 

 ing, by the two pairs of curves, the relative 

 position of the reflected and unreflected por- 



tibflsof ahv MIetilidulation- at tM'0's»t?cdsst^'itf 

 &mt>iy ai<# ri'siJ*,' hy sdind^i lisefe dravtri 

 aertoss, the pjittfe vt'here the intervals of ret 

 tardation are in arithmeticid pro2;ressioH, 

 and where similar colours' will be exhibited at 

 different distances from the inflecting sub- 

 stance. The lesult agrees sufficiently with 

 the observations of Newton's thiril book, and 

 with those of later writers. But 1 do nW 

 consider the existence of such an atmosphere 

 as necessar}' tb the explanation of the phe- 

 nomena; the siinple opinion ofGrimaldi andi 

 Hooke, who supposed that inflection arises 

 from the natural tendency of light to diverge, 

 appearing equally probable. 



Proposition i.x. liadiant Light consists 

 in Undulations of the luminifirous Ether. 



This proposition is the general conclusion 

 from all the preceding; and it is conceived 

 that they conspire to prove it, in as satisfac- 

 tory a manner, as can possibly be exjiccted 

 from the nature of the subject, [t is clearly 

 granted by Newton, that there are ethereal 

 . undulations, y6t he denies that they constitute 

 light; but it is shown in the Corollaries 

 oi' the last Proiwsition, that all cases of 

 the increase or diminution of light are refer- 

 able to an increase or diminution of such un- 

 dulations, and that all tiie affections, to 

 which the undulations would be liable, are 

 distinctly visible in the phenomena of light; 

 it may therefore be very logically inferred, 

 that the undulations are light. 



A few detached remarks will serve to ob- 

 viate some objections which may be raised 

 against this theory. 



1. Newton has advanced the singular re- 

 fraction of the Iceland crystal, as an argu- 

 ment that the particles of light must be pro- 



