NOTICES FROM THE LICK OBSERVATORY.

PREPARED BY MEMBERS OF THE STAFF.

ENLARGEMENTS OF THE LICK OBSERVATORY PHOTOGRAPHS OF THE MOON.

Mr. A. L. COLTON, Assistant Astronomer, is now devoting a large part of his time to enlarging the original negatives of the Moon, previously obtained. One series of enlargements is made on 8 x 10 plates to a scale of Moon's diameter 3 Paris feet. Quite a number of plates to this scale have already been obtained, and some of them have been carefully studied in connection with MAEDLER'S, LOHRMANN'S and SCHMIDT's maps. In one respect (only) the latter maps seem to be superior to the enlarged plates: They give extremely small craters which are often not to be clearly distinguished on the photographs. The reason of this is, I think, that the rapid plates on which the original negatives (in the focus) must be taken, have a comparatively coarse grain; so that objects of small angular dimensions are confounded with the grain of the sensitive film. If quick plates with a fine grain can be manufactured, as seems probable, the negatives in the focus will be much improved in this regard, for experiment has shown that the deficiency in this matter is almost solely due to the plates themselves, and is not a deficiency of telescopic definition. In every other respect a transparency on glass enlarged from our negatives must be pronounced to be vastly superior to the maps. The maps give only the baldest outlines of the lunar topography and even these are given in a strictly conventional way, without any true plastic effect, while the transparencies are very satisfactory in all respects, both as maps and as pictures. The wonderfully beautiful drawings of lunar landscapes enlarged from our negatives by Professor WEINEK, reproduced by heliogravure, are also very satisfying and marvelously accurate, as I have had frequent occasion to know through pains

taking comparisons between them and the negatives. One of these (Mare Crisium) will be printed in an early number of our Publications. Still, it is not possible for any artist, no matter how diligent, to produce such representations of the whole of the Moon, in all its varying aspects of illumination and libration, for the simple reason that a life is not long enough. Professor WEINEK is obliged to spend from 100 to 200 hours in making a single plate. It is necessary, therefore, to depend upon the Camera for the general representation, leaving specially interesting regions to be specially studied by the artist.

Some portions of the Moon have already been enlarged to a scale of the diameter 6 Paris feet (the same as that of SCHMIDT'S maps), with great success, and a few craters have been enlarged far more than this. It is hoped to reproduce some of these shortly. The general result of our work of enlargement so far is that each plate brings a multitude of new features to light, and that these are all exhibited in their true relations. If anyone is seeking to make a catalogue of new rills or new craters, such negatives afford a simply boundless field. But, better than this, they give a perfectly true picture of the lunar surface, in which there is no need for conventional signs for slopes or hollows. It requires but a moment to compare

such a picture taken last year with the Moon itself, and to decide whether or no a change has occurred. I need not say that such a decision based on the comparison of maps and Moon is very difficult and unsatisfying. If lunar geology is ever to be

studied in detail, it must have such pictures as a basis. The chief problem yet to be solved is how to reproduce our negatives in large editions, so that they may be as available to others as they are to us. The heliogravure process is almost entirely satisfactory in this respect, but it is, unfortunately, very expensive. Another most serious practical difficulty is, the coarse grain of the sensitive films employed in making the original plates.

The results already reached, in spite of these drawbacks, entirely confirm my previously expressed opinion that the future of lunar studies must rest, in the main, upon photographs, and that the photographs already available to science are of more value than all preceding data obtained by other methods.

It is hoped to print in these Publications, during 1893, some reproductions of our enlargements, which will justify this judgment.

E. S. H.