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[Proceedings of the Royal Society, A, Vol. xc. pp. 219225, 1914.]
IN a short paper "On the Diffraction of Light by Particles Comparable with the Wave-lengthf," Keen and Porter describe curious observations upon the intensity and colour of the light transmitted through small particles of precipitated sulphur, while still in a state of suspension, when the size of the particles is comparable with, or decidedly larger than, the wave-length of the light. The particles principally concerned in their experiments appear to have decidedly exceeded those dealt with in a recent paper J, where the calculations were pushed only to the point where the circumference of the sphere is 2'25 X. The authors cited give as the size of the particles, when the intensity of the light passing through was a minimum, 6 /A to 10 /j,, that is over 10 wave-lengths of yellow light, and they point out the desirability of extending the theory to larger spheres.
Th,e calculations referred to related to the particular case where the (relative) refractive index of the spherical obstacles is 1*5. This value was chosen in order to bring out the peculiar polarisation phenomena observed in the diffracted light at angles in the neighbourhood of 90, and as not inappropriate to experiments upon particles of high index suspended in water. I remarked that the extension of the calculations to greater particles would be of interest, but that the arithmetical work would rapidly become heavy.
There is, however, another particular case of a more tractable character, viz., when the relative refractive index is small*; and although it may not be the one we should prefer, its discussion is of interest and would be expected
* [1914.    It would have been in better accordance -with usage to have said " of Eelative Index differing little from Unity."]
f Roy. Sac. Proc. A, Vol. LXXXIX. p. 370 (1913).
J Roy. Soc. Proc. A, Vol. LXXXIV. p. 25 (1910); Scientific Papers, Vol. v. p. 547. requires that