Pleochroic Halos


IN a recent reference to the subject of pleochroic halos (Phil. Mag., February, 1910) I stated that the outer edge of a corona might present an appearance suggesting an actual accentuation or deepening of the coloration, in accordance with the fact observed by Bragg and others that the ionisation of the α ray increased just before the limits of its ionising range was attained. For certain stages of development of the halo, this observation I have more recently confirmed beyond doubt. Like other structural particulars referred to in my paper, this too becomes obliterated by over-exposure. In one case (in the lithia mica of Zinnwald) a stage of development has been found in which the extreme outer border of the corona is the sole visible part of that structure, the appearance presented being that of a detached, very delicate, ring of perfect regularity surrounding the central halo, a space showing no definitely visible ionisation intervening. The outer ring has a radius of 0.0344 mm., and the inner halo a radius of 0.0191 mm. The outer ring is of about normal radial dimensions; the inner radius is that corresponding to under-exposure to the slower moving α particles. Reference to Bragg's curves (Phil. Mag., September, 1905) will more fully explain.

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JOLY, J. Pleochroic Halos. Nature 82, 428 (1910).

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