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



are somewhat purer and more inclined to orange. The curve 

 ultimately circles round closer and closer to the point X, a 

 very pale orange-yellow. 



The custom of speaking of the successive diffraction spectra 

 is apt to lead to the impression that each spectrum is purest 

 in the middle when it does not overlap its neighbours. In 

 the colours of the first two orders the exact contrary is the 

 fact. A better idea of the phenomenon is arrived at by con- 

 sidering the wave-lengths that are absent ; in other words, by 

 considering the dark bands in the spectrum into which each 

 colour could be drawn out. The fine yellows of the first two 

 orders are due to the upper part of the spectrum being nearly 

 quenched by broad dark bands, which as they proceed down 

 the spectrum give the blues of the second and third orders. 

 Before we can obtain a good green we must have two bands 

 to blot out both ends of the spectrum. This occurs at 1330 

 and 1830. 



Fraunhofer (Verdet § 70) using white light measured the 

 deviations of the red bands in the diffraction-image of a slit, 

 and, finding they were in the ratio 1:2:3 . . , thought he 

 had discovered the law for the successive maxima of homo- 

 geneous light. The complete explanation of this may be seen 

 in the diagram ; for the points 500, 1000, and 1500, corre- 

 sponding to the absence of wave-lengths in the neighbourhood 

 of 500, lie almost on the line from W to the red corner. 

 The fourth red was probably not measured by Fraunhofer. 

 The real maxima for wave-length 631 are at the points 0, 900, 

 1550, 2190. 



Maxwell's colour diagram gives us complete information as 

 to the hue and depth of each tint, but is silent as to the bright- 

 ness ; and with cloud colours, which are necessarily more or 

 less contaminated with white light, the brightness is of great 

 importance. It is clear, too, that the power of withstanding 

 contamination depends on the depth as much as on the 

 brightness. It occurred to me, therefore, that it would be 

 instructive to draw a curve with retardations as abscissas, in 

 which the ordinates should depend on both these qualities, 

 and should represent what I will call the brilliancy of the 

 colour. I have used the following principles : — (1) the bril- 

 liancy of white light is zero ; (2) the brilliancy of standard 

 red light is reckoned equal to that of standard green or violet 

 light, when they are in the proportion in which they occur in 

 white light ; (3) the brilliancy of any colour which is com- 

 posed of two standard colours is equal to the brilliancy of the 

 more brilliant component. The third principle ensures that the 

 brilliancy of complementary colours should be equal. As an 



