LORD RAYLEIGH ON THE COLOURS OF THIN PLATES. 167 



description as nearly as may be in his own words, but adapted to the more 

 convenient notation followed by Herschel : — 



" The red of the fourth order was also dilute and dirty, but not so much as 

 the former three ; after that succeeded little or no yellow, but a copious green 

 (fourth order), which at first inclined a little to yellow, and then became a pretty 

 brisque and good willow-green, and afterwards changed to a bluish colour ; 

 but there succeeded neither blue nor violet. 



"The red of the third order inclined very much to purple, and afterwards 

 became more bright and brisque, but yet not very pure. This was succeeded 

 with a very bright and intense yellow, which was but little in quantity and soon 

 changed to green ; but that green was copious and something more pure, deep 

 and lively than the former green. After that followed an excellent blue of a 

 bright sky colour (third order), and then a purple, which was less in quantity 

 than the blue, and much inclined to red. 



" The red of the second order was at first a very fair and lively scarlet, and 

 soon after of a brighter colour, being very pure and brisque, and the best of all 

 the reds. Then after a lively orange followed an intense bright and copious 

 yellow, which was also the best of all the yellows ; and this changed first to a 

 greenish-yellow and then to a greenish-blue ; but the green between the yellow 

 and the blue was very little and dilute, seeming rather a greenish- white than a 

 green. The blue which succeeded became very good, and of a very fair bright 

 sky-colour, but yet something inferior to the former blue ; and the violet was 

 intense and deep, with little or no redness in it, and less in quantity than the 

 blue. 



" In the last red appeared a tincture of scarlet next to violet, which soon 

 changed to a brighter colour, inclining to an orange ; and the yellow which 

 followed was at first pretty good and lively, but afterwards it grew more dilute, 

 until by degrees it ended in perfect whiteness."* 



§ 9. Some small discrepancies in the descriptions of Newton and Herschel 

 probably depend upon ambiguities in the use of colour names. In the rings of 

 high order what Newton calls blue, Herschel describes as bluish-green. Both 

 observers remark upon the poverty of the green of the second order, but the 

 diagram shows that it is superior to that of the fifth order. Neither Newton 

 nor Herschel seem to have done full justice to the green of the fourth order, 

 which at its best rivals closely the corresponding colour of the third order. 

 My own observations are in accordance with the teaching of the diagram, which 

 shows, moreover, that as we depart from retardation 6800 the colour of the 

 fourth order rapidly deteriorates by admixture with white, while the colours of 

 the third order in the neighbourhood of 4800 retain their purity as they change 

 in hue. 



* Newton's Opticks, 1704, book ii. p. 21. 



