166 LORD RAYLEIGH ON THE COLOURS OF THIN PLATES. 



very closely approached, a contrary curvature sets in, so that the earlier reds 

 are more blue than the later. The curve then bends round on the yellow side 

 of white, until it attains a rather feeble blue-green at 9000. 



§ 7. It will be interesting to compare the diagram with descriptions by 

 previous writers of Newton's scale of colours. In his article on Light in the 

 Encyclopaedia Met ropolitana (1830), Sir John Herschel says: — "The colours, 

 whatever glasses be used, provided the incident light be white, always succeed 

 each other in the same order ; that is, beginning with the central black 

 spot as follows : — 



" First ring, or first order of colours, — Black, very faint blue, brilliant white, 

 yelloiv, orange, red. 



" Second ring, or second order, — Dark purple or rather violet, violet, blue, 

 green (very imperfect, a yellow-green), vivid yellow, crimson-red. 



"Third ring, or third order, — Purple, blue, rich grass-green, fine yellow, pink, 

 crimson. - 



"Fourth ring, or fourth order, — Green (dull and bluish) , pale yellowish-pink, red. 

 " Fifth ring, or fifth order, — Pale bluish-green, white, pink. 

 " Sixth ring, or sixth order, — Pale blue-green, pale pink. 

 "Seventh ring, or seventh order, — Very pale bluish-green, very pale pink. 

 "After these the colours become so pale that they can scarcely be dis- 

 tinguished from white. 



" On these we may remark, that the green of the third order is the only one 

 which is a pure and full colour, that of the second being hardly perceptible, 

 and of the fourth comparatively dull and verging to an apple-green; the yellow 

 of the second and third orders are both rich colours, but that of the second is 

 especially rich and splendid ; that of the first being a fiery tint passing into 

 orange. The blue of the first order is so faint as to be scarce sensible, that of 

 the second is rich and full, but that of the third much inferior ; the red of the 

 first order hardly deserves the name— it is a dull brick-colour ; that of the 

 second is rich and full, as is also that of the third ; but they all verge to 

 crimson, nor does any pure scarlet or prismatic red occur in the whole series." 



Herschel's observations were made in the usual way with glass lenses, — a 

 course convenient in respect of measurement of thicknesses, but incapable of 

 doing justice to the colours, in consequence of the contamination with white 

 light reflected at the upper surface of the upper plate and at the lower surface 

 of the lower plate. The latter reflection should at any rate be got rid of by 

 using a glass, either opaque, or blackened at the hind surface. 



§ 8. For his description Newton used the soap-bubble, " because the Colours 

 of these Bubbles were more extended and lively than those of the Air thin'd 

 between two Glasses, and so more easy to be distinguished." He takes the 

 colours in the reverse order, beginning with large retardations. I give his 



