Abstract
Abstract A difficult problem in observational optical astronomy is to measure the extremely small angles subtended at the Earth by the discs of stars. There are three techniques; Michelson's stellar interferometer, the stellar intensity interferometer at Narrabri, and lunar occultations. Michelson's interferometer has been used to measure a few cool giants and supergiants but, for technical reasons, attempts to develop it further have not, so far, proved successful. The stellar intensity interferometer is capable of very high resolving power and is unaffected by atmospheric seeing. It has been used to make all the existing measurements on hot stars. The present instrument at Narrabri in Australia is limited to stars brighter than magnitude + 2.5 and hotter than 6500°K (spectral type K5). A larger instrument could be built which would open up many new and interesting possibilities. The method of lunar occultations is comparatively simple and inexpensive. The accuracy of the results is still uncertain becau...
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