Abstract
Direct optical measurements of asteroid diameters obtained by telescopic observations are scarce. Although several adequate instrumentations and techniques are available for the purpose, they have not been used. The importance of these determinations should be stressed for the attention of observers.The filar micrometer was used only by one observer during the last century, and no additional measures have been made since. This was in 1894 and 1895 when E. Barnard (1902) used the 90 cm refractor of Lick Observatory, and the 100 cm refractor of Yerkes Observatory. The results are as follows:Ceres: apparent diameter at 1 AU: l˝.060, or 770 km (28 nights)Pallas: apparent diameter at 1 AU: 0˝.675, or 490 km (5 nights) Juno: apparent diameter at 1 AU: 0˝.266, or 195 km (5 nights)Vesta: apparent diameter at 1 AU: 0˝.531, or 390 km (21 nights)
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