Registration of Solar Prominences.—In a recent issue (No. 88) of the Bulletin of the Kodaikanal Observatory -a biannual publication containing current observations of solar prominences and disc phenomena recorded spectroscopically at Kodaikanal—the Director, Dr. T. Royds, directs attention to the publication of additional data of prominences photographed in hydrogen light (Ha). There are two spectroheliographs in daily use at this observatory, one of which is used for obtaining composite photographs showing the disc markings and the prominences at the sun's limbs in calcium light (K), whilst the other instrument is employed for registering the disc markings in hydrogen (Ha) light. Owing to the increased speed of panchromatic plates, it has been found possible to include in the daily programme since Jan. 1, 1929, the registration of the prominences in hydrogen light. Dr. Royds finds that the mean daily areas of Ha prominences in 1929 are considerably less, about 54 per cent, than those of the calcium prominences. This, he points out, is not necessarily to be interpreted as evidence that the hydrogen prominences are less extensive or less high than calcium prominences, for there are numerous examples where individual prominences are identical in shape, height, and area in the Ha and the K photographs. There is, however, considerable evidence that in the fainter and more scattered parts of K prominences the Ha counterpart is relatively much fainter when compared with the brighter parts of the prominence. This is not a photographic effect caused by the under-exposure of the Ha plate, for whilst the main part of a prominence may be stronger in the Ha photograph than in the calcium, the reverse is often true in the fainter parts of the same prominence. The exact relations between the relative intensities in different parts of Ha and K prominences require further study, and it is very satisfactory that data for this purpose will be accumulated at Kodaikanal.