LORD RAYLEIGH ON THE COLOURS OF THIN PLATES. 169 



for the increasing poverty of the real colour. Much, again, depends upon the 

 rapidity with which differences occur with varying retardation. When Newton 

 speaks of the yellow of the second order as copious, he refers (I imagine) 

 rather to the width of the band than to the brightness of the light. The 

 diagram gives important information on this subject also. Compare, for 

 example, in the first order, the change from 1500 to 1755, with that from 1755 

 to 1846 or 1951. The rapidity of the change in the latter interval is the 

 foundation of the usefulness of the " transition-tint " in polarimetric work. If 

 we wish to compare the rates of progress in different orders, we must dis- 

 tinguish according as we contemplate sensitiveness to small absolute, or to 

 small relative, variations of retardation. 



§ 11. The points of intersection of the curve are of interest, as corresponding 

 to colours obtainable with two different thicknesses. The first that presents 

 itself is the yellow, common to the first and second order. The table shows 

 that the latter is the brighter. In the second and third orders the similar 

 colours differ but little in brightness. One occurs in the blue and another in 

 the greenish-yellow. Nor is there much difference of brightness between 

 the otherwise nearly identical greens of the third and fourth orders. It follows 

 that if observers are able to distinguish in all cases which order of colours they 

 are dealing with, it must be by reference to a sequence, rather than by estima- 

 tion of a single colour. 



§ 12. With respect to the absolute retardations or thicknesses at which the 

 various colours are formed, careful observations have been made by Reinold 

 and Rucker.* For comparison with their results I will take the green of the 

 fourth order at 6800. In air at perpendicular incidence, this answers to a 

 thickness of 340 x 10" 5 Paris inches, or 919 x 10~ 5 cm. The numbers in their 

 Table (p. 456), Column V., are 



Green, ...... 8 - 41 



8-93 



Yellow-green, ...... 9*64 



so that the agreement is pretty good. I would remark in passing that the 

 diagram does not recognise a yellow-green of this order ; but the appearance 

 of such may perhaps be explained by contrast. 



§ 13. The series of colours complementary to those of Table III. are found 

 by subtraction of the numbers there given from those representative of white, 

 viz., 397, 6'52, 6*46, respectively. The resulting numbers are exhibited in Table 

 IV., in which the first entry for zero retardation corresponds to the full white, t 



* "On the Electrical Resistance of Thin Liquid Films, with a Revision of Newton's Table of 

 Colours," Phil. Trans., 1881. 



f In comparing with Table III., it should be remembered that the numbers there given under the 

 head of V = are relative only, the true values being infinitely small. 



VOL. XXXIII. PART I. Z 



