The method of point-group determination from convergent-beam electron diffraction patterns has been established by Buxton, Eades, Steeds & Rackham [Philos. Trans. R. Soc. London (1976), 281, 171-194]. However, Table 2 given by them is inconvenient for practical purposes, since many symmetries of the dark-field and +G dark-field patterns are not given and are left for the reader's consideration. The table is improved and completed with the help of some new symmetry symbols and illustration of symmetries. The new table makes the point-group determination easy and quick. The symmetries of the symmetrical manybeam convergent-beam electron diffraction patterns have been studied by Tinnappel [PhD Thesis (1975), Tech. Univ. Berlin l using group theory. It is shown that the graphical method used by Buxton et al. can reveal the symmetries of these patterns. A method of * Present address: Hitachi Research Laboratory, Hitachi 319-12, Japan. point-group determination which uses three types of symmetrical many-beam patterns, the hexagonal sixbeam, square four-beam and rectangular four-beam patterns, is described. This method requires only one photograph in determining most diffraction groups. This fact means that the method is more convenient and reliable than that of Buxton et al., since their method requires two or three photographs for most cases. Experimental results which verify the theoretical ones are given. The characteristic features of the symmetrical many-beam method are discussed.

Tables of dynamic extinction lines which occur in convergent-beam electron diffraction patterns (GM lines) are given for all the space groups on the basis of the rules given by Gjonnes & Moodie [Acta Cryst.

A convergent-beam electron diffraction study has revealed that the space group of the room temperature phase of La2Ti2O7 is P21. A dark-field electron microscopy has made clear a twinned domain structure in this phase. C BED patterns obtained at above 1053 K were compatible with the space group Cmc21. An incommensurate phase exists between 993 K and 1053 K. The incommensurate reflections are characterized with the vector q̄=((1/2)-δ)ā 0 * from the fundamental reflections of the high temperature phase, a 0 * being the reciprocal lattice parameter of the high temperature phase. The value of δ increases from 0.015 to 0.017 as the temperature is raised. The transition between the room temperature phase and the incommensurate phase is of the first order, and that between the latter phase and the high temperature phase is of the second order.

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Quartcr century has passed since LED light source wus put te practical use in 賦 gh1ing applications ， and it is now used in almost all of the lighting equipment ． Paradigm shift of the light source has a major impact 【 o the optical system such as lens and reflector 、 In this feature article， we described the transition of the optical system by change of the light source ．