OF NEWTON'S OPTICS. 



nesses Rr, Rr', R? 1 ", Sax. the reflection 

 of red light is most intense, the ray being 

 at these points in a fit of easy reflection. 

 From the first entrance of the red ray at 

 the plate A Y until it reaches r t its dis- 

 position to be reflected is increasing, and 

 consequently, the curve from R to r 

 may be so shaped, that its increasing 

 perpendicular distance from R r may be 

 proportionate to the quantity of red light 

 reflected at each increasing thickness. 

 If this be done, the perpendicular dis- 

 tance of the curve from r will represent 

 the quantity of red light reflected when 

 the ray attains its first fit of easy reflec- 

 tion. As the ray passes from r to R' it 

 gradually changes its phase, and the 

 reflected light constantly decreases like 

 the distance of the curve from r R', until 

 at length, like that distance, it dwindles 

 into nothing at R', the entire light being 

 here transmitted, the ray having attained 

 its fit of easy transmission. The same 

 process is repeated as the ray passes 

 from R' to R", from R" to R'", and 

 so on. 



It thus appears, that the quantity of 

 red light reflected by the plate of air 

 intervening between the two plates of 

 glass may be exhibited. Take R p 

 equal to the distance between the plates, 

 or the thickness of the plate of air. 

 Draw p m perpendicular to Rp and 

 meeting the curve at m. Then p m will 

 bear the same proportion to ra as the 

 quantity of red light reflected at the 

 thickness R;; bears to the quantity re- 

 reflected at the thickness R r when the 

 ray is in a fit of easy reflection. It is 

 evident that the quantity of red light 

 reflected at any other thickness may be 

 similarly found. 



Let O O' be taken to represent the 

 interval of the fits of orange light, and 

 let this interval be repeated O' O", 

 O" O'", See., as in the former case. Let 

 a curve be drawn as before, touching 

 the parallel at the points which mark 

 the fits of easy transmission, and such 

 that its distance from the parallel will 

 always be proportionate to the quantity 

 of orange light reflected at each thick- 

 ness of the plate of air. The other 

 curves are to be drawn in the same 

 manner, the distances YY', GG', BB', 

 1 1', V V' representing the intervals of 

 the fits of the yellow, green, blue, in- 

 digo, and violet lights respectively. It 

 is evident that, by this scale, the quan- 

 tity of light of each colour which is re- 

 flected at any given thickness may 

 always be exhibited, and the colours 



which compose the tint, perceived by 

 reflection, may thus be determined, both 

 in quantity and quality* 



It will be observed, that the intervals 

 R R', O O', Y Y', &c., continually dimi- 

 nish in passing from the red to the 

 violet light. This is conformable to 

 what has been explained ; the interval 

 of the fits being shorter for the more 

 refrangible the lights. It is to this 

 circumstance that the coloured rings 

 between the lenses is owing ; for if the 

 fits of all the component parts of light 

 were equal, the rings would be alter- 

 nately white and black. 



To explain more fully the manner of 

 determining a tint corresponding to a 

 given thickness, let the line AX be 

 divided at 1,2, 3, &c. Let A 1 be a 

 thickness much less than R r, half the 

 interval of the fits of red light. Through 

 1 draw a line parallel to AY, and 

 crossing all the curves. The parts of 

 this line intercepted between each curve 

 and the corresponding parallel to A X, 

 express the quantities of the respective 

 colours reflected by the air at this thick- 

 ness. It thus appears, by inspection, 

 that the quantities of red and orange 

 are small ; of yellow not much more, 

 but that the proportion rapidly increases 

 as we approach the blue and violet. 

 The excess of light of a bluish tint, 

 therefore, which enters the reflected 

 light, will give that light a corresponding 

 character. If the thickness of the plate 

 of air be much less than A 1, the lines 

 representing the reflected light will gra- 

 dually disappear, and no light will be 

 reflected. Accordingly, it was found in 

 the experiment with the glass lenses 

 described in (54), that at the centre, 

 where the glasses were in contact, and 

 for a small distance round it, a black 

 spot was perceivable, arising from the 

 absence of reflected light. In this case, 

 the air immediately around the centre 

 was too thin to reflect the light in any 

 sensible quantity, as appears by the 

 scale which we are now describing. 



Let a parallel to AY be drawn 

 through 2. The lines which now repre- 

 sent the reflected lights are nearly equal. 

 The red tints have not reached their 

 maxima, and the blue tints have passed 

 theirs. The intermixture of these pro- 

 duce a brilliant white. 



Referring again to the experiments 

 with the glass, we found that the blw 

 ring which immediately succeeded tht 

 central black spot was followed by i 

 ring of splendid white. Here, then, the 



