282 Mr. J. C. M c Connel on Diffraction- Colours, 



The white obtained by superimposing unit widths at the 

 ten points is given by 



W = 2-01(24) +3-27(44) +3-22(68). 



The chief defect of this table is the omission of the red 

 corner (24). This has been in great measure allowed for by 

 modifying the coefficients for (20) and (28). At the same 

 time the white was brought to practical coincidence with the 

 white of Lord Rayleigh's table. 



On the diagram (PI. X.) are marked the positions of sixteen 

 points equidistant in the prismatic spectrum, from 20 to 80 on 

 Maxwell's scale, with the corresponding wave-lengths. These 

 lie, for the most part, outside the triangle. Rood has deter- 

 mined the places in the spectrum which, when diluted with 

 a suitable amount of white, match the colours of certain 

 pigments (' Modern Chromatics/ p. 38). I had no data for 

 marking the true position of the pigments on the diagram, but 

 their hues (i. e. the radii from white on which they lie) are 

 indicated. I have also divided the diagram into five parts, — 

 blue, green, yellow, red, and purple, chiefly in order to name 

 the hues in the " brilliancy " curves described below. In this 

 I have been mainly guided by Rood's l Modern Chromatics.' 

 On the spectral colours his statements are definite. But the 

 limits of purple, founded on considerations of complimentary 

 colours, are more doubtful. The estimation of hue depends 

 greatly on the brightness of the light and the purity of the 

 colour ; and of course, at the best, the lines of division must 

 be rather indefinite. The pure yellow in the spectrum is a 

 very narrow band ; so my yellow division consists mainly of 

 orange-yellow and greenish yellow. 



In the previous section I have shown that to find the 

 colours in the principal directions of the diffraction-pattern of 

 a rectangular aperture, the proper factor to multiply each of 

 the constituents of sunlight before compounding them is 



sin 2 -=p For the colours of thin plates the appropriate 



factor, " strictly applicable only to a plate of air bounded by 

 media of small refrangibility," but practically sufficient for all 



ordinary cases, is sin 2 ——. Thus identical colours are found 



in the two cases, whenever the " retardation " V for the thin 

 plate is equal to the extreme retardation a£/f of light from one 

 edge of the aperture relative to light from the other. The 

 dotted curve (copied from Lord Rayleigh's) represents these 

 colours, and the small figures at the side are values of a!j/f 

 expressed, like the wave-lengths, in millionths of a millimetre. 



