OF NEWTON'S OPTICS. 



49 



highest point of the curve Y Y' will 

 correspond to the lowest of G G', so 

 that although no green light of the re- 

 fransribility of G be reflected, yet light 

 of the same colour, but of the refrangi- 

 bility of Y, will be reflected in abun- 

 dance ; so that at considerable thick- 

 nesses, green light, of some degrees of 

 refrangibility, must always be reflected. 

 Now, what we have to observe of green 

 light is equally applicable to light of all 

 other colours : so that it follows, that 

 light of all colours will be both reflected 

 and transmitted at considerable thick- 

 nesses. 



(63.) Since a mixture of lights of all 

 colours constitutes white light, it follows 

 that the light reflected and transmitted by 

 a thick plate of air will always be white ; 

 but since some is transmitted, and some 

 reflected, neither the reflected nor trans- 

 mitted light will be so intense as the 

 lights which are reflected and trans- 

 mitted by very thin plates. 



The recurrence of the fits of lights of 

 nearly equal refrangibility do not sud- 

 denly attain the state which we have 

 described. They approach it gradually. 

 Hence we may account for the dilute 

 appearances of .the colours reflected as 

 the thickness of the air increases, as is 

 perceived by observing the rings of 

 colour at a considerable distance from 

 the centre of the lenses. They become 

 faint by degrees, and finally disap- 

 pear. 



(64.) Another circumstance attending 

 the phenomena of the rings is accounted 

 for on the grounds to which we havejust 

 adverted. When homogeneous light 

 was projected on the lens, the rings 

 appeared in much greater numbers, and 

 more distinctly defined than when com- 



pound solar light was used. In fact, in 

 this case their number seemed quite 

 interminable. The colours were here 

 not diluted and neutralized by the ad- 

 mixture of rays of all hues, as we have 

 shown to be the case when white light 

 was projected on the lenses. 



(65.) Seeing that the interval of the 

 fits changed with every change of refran- 

 gibility of the light, the problem to de- 

 termine the relation between the interval 

 of the fits and the degree of refrangibility 

 obviously presented itself. This, how- 

 ever, Newton failed to solve. He fan- 

 cied that the intervals of the fits of the 

 seven coloured lights bore an analogy 

 to the lengths of a string which sound 

 the seven musical notes. 



(66.) Hitherto we have conceived the 

 light to fall perpendicularly on the plate 

 of air, and therefore to be reflected per- 

 pendicularly by it. If the light enter 

 the air obliquely to its surface, it will be 

 reflected at the same obliquity. In tbis 

 case the light is affected by the oblique 

 incidence in a singular manner. The 

 intervaj. of its fits is lengthened, and 

 bears a certain proportion to the length 

 of the interval when perpendicular. 

 Newton succeeded in detecting this pro- 

 portion, and in showing how the length 

 of the fit depended on the obliquity of 

 the light. 



Although the phraseology of mathe- 

 matics may be considered in some de- 

 gree necessary to express this curious 

 relation, yet we hope to render it intel- 

 ligible to the general reader, if a mode- 

 rate portion of attention be given to the 

 following explanation. Let the line 

 PD (fig. 46.) represent the interval of 

 the fits of any species of homogeneous 

 light incident perpendicularly on a thin 



Fig. 4 7 . Cmo P 



