Haptic interfaces enable person‐machine communication through touch, and most commonly, in response to user movements. We comment on a distinct property of haptic interfaces, that of providing for simultaneous information exchange between a user and a machine. We also comment on the fact that, like other kinds of displays, they can take advantage of both the strengths and the limitations of human perception. The paper then proceeds with a description of the components and the modus operandi of haptic interfaces, followed by a list of current and prospective applications and a discussion of a cross‐section of current device designs.

A method is described for finding the peak limits of a Bragg reflection from an analysis of its profile. A `window' of ± 0.67σ[ϕ)(x)] is set up on each ordinate ϕ(x) in the tails of the profile and the pattern of the following ordinates is observed through the window. As the ordinate moves from the peak to the background the pattern changes and the limits of the peak may be recognized. Bragg reflections from three crystals have been analysed by this method and the limits found were generally wider than those determined by the method of Lehmann & Larsen [Acta Cryst. (1974), A30, 580–584] applied to the same data. The value of σ(I)/I was near the minimum and similar to the value from the corrected Lehmann–Larsen limits. The behaviour of the methods is also compared by representing the reflections by equivalent Gaussian profiles, and it is found that >99% of the Gaussian peak is within the limits found by the present method.

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1"he crystal and molecular structure of trichlorotrimethylaminealuminum(III) [A1CI3.N(CH3)3] has been determined by a three-dimensional X-ray analysis and is shown to be monomeric. The crystals are monoclinic with a --6.81 +_ 0.02, b = 10.66 _+ 0.03, c = 7.31 +_ 0.02/~; p--118°0 , _+ 5'; space group P21 with Z= 2. The aluminum-nitrogen distance is 1.96/~.

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