WITH regard to the above crystals, described by Prof. Stokes in NATURE (April 16, p. 565), I should like to suggest, with some diffidence, that the colours may be due, not to a continuous hemitropic crystal-film, but to a series of fine tubular cavities ranged parallel to each other between the two main portions of the crystal, such as not unfrequently occur on a large scale in Iceland spar, and appear to be due to bad fitting (so to speak) of hemitrope-films on the rest of the crystal (see Groth's “Physikalische Krystallographie,” p. 441).