It does not seem to the writer practicable to abolish the system of names at present applied to the chief lunar craters. A supplementary system of nomenclature for designating the minor points, however, seems to him very desirable. He would therefore make the following suggestions to the Committee on Nomenclature. A series of photographs should be taken, or, if practicable, negatives already taken should be used. These should be enlarged through a reticule to a uniform scale of 400 mm. to the Moon's diameter. The positives should be on plates measuring 20 x 25 cm. The enlargements should be made through a reticule of squares, each square measuring 20 mm. on a side. These positives would form a set of standards which should be safely stored. From them negatives should be printed which could be furnished to engravers, photographers, or the public. The reticule should be oriented parallel to the Moon's axis.

Since it is desirable to get all the detail possible, and since the maximum detail is shown only near the terminator, it is proposed

to use photographs taken at eight different ages of the Moon, one set representing the northern hemisphere and the other the southern. This will give us sixteen different photographs. Each of these should be selected in as favourable a libration as possible. Besides the lunar detail which shows near the terminator, there is an entirely different kind of detail, consisting of bright and dark surfaces, and of bright points which show only when remote from the terminator. Six plates should be taken to show this detail, three for each hemisphere. These would be taken when the terminator is distant, but not necessarily at full moon. This would give us a class of detail which has not been so carefully studied as that at the terminator, and for this reason is of all the more importance.

We thus have in all twenty-two standard plates. While it would be desirable to have an atlas engraved from them which

since a

could be issued to the public, this would not be necessary, photographic edition of the whole twenty-two would not be very expensive. It should be possible for those interested to buy photographs of a single region.

The positive standards should be lettered in capitals running from the western to the eastern limb, and in small letters running from north to south. Twenty-four letters should be used, j and w being omitted. Around the selected region of each standard a heavy black line should be drawn, as shown in the figure, and the regions outside of this should not be used. The selected regions should overlap slightly on the different plates, which the use of twenty-four letters will permit. Otherwise only twenty letters would be needed. Beginning at the western limb, two capitals should be assigned to the first plate, three to the second, and three to the third. Four capitals will be assigned to the fourth plate, as

shown in the sketch, and the same arrangement will apply to the other quadrants.

Any area of the Moon's surface can now be indicated by two letters. Thus Kq will be found on only a single sheet of the atlas, and its exact area will be there clearly shown. The six sheets taken away from the terminator, under high illumination, could be indicated by the use of Greek letters.

The sides of the squares may be divided to tenths by the eye, and the position of any point within the square indicated thus, "Kq 63," indicating that the point measures 6 way from the side between J and K, and 3 way from the bottom of the square. Should still greater accuracy of designation be required, a millimetre scale could be used, and the point designated thus, "Kq 65, 37." The squares measure approximately 100" on a side, so that this system should give us all the accuracy needed.

A double catalogue would go with the atlas, one part giving the old nomenclature of every point hitherto named in terms of the new, and the other giving the new in terms of the old. It is not intended that this nomenclature should supersede the older one; the idea is simply to furnish one which shall be purely supplementary, and absolutely free from ambiguity. It could be made readily accessible to all, so that for purely scientific work anyone could use it who chose to do so; and since the older nomenclature would also be given in the catalogue, no one need feel compelled to use the supplementary system who did not like it.

1907 November 8.

WILLIAM H. PICKERING.

MR. SAUNDER'S PROPOSALS.

I have already expressed my views as to the best method of dealing with the problem presented by the present condition of Lunar Nomenclature in R.A.S. Monthly Notices, lxvi. p. 41, and in a note printed in view of the submission of the question to the Vienna Congress. It might, however, be convenient if I summarise them here.

I am not in favour of any radical change. So much work of permanent value has been published in terms of the old nomenclature that it can never be entirely superseded, and to create another, to be used concurrently with it, would, in my opinion, only increase the confusion.

The present nomenclature, however, does require careful revision in order to ensure that

(1) No two formations should have the same name.

(2) No formation should have more than one name.

In assigning names for future use I would suggest the following general rules

(1) That Mädler's principal names be retained.

(2) That where Mädler has not given principal names, as little alteration as possible be made in names now generally accepted. These rules should not prevent the Committee from rejecting a name altogether where none seems required, or from introducing a new one where one would be advantageous.

;

(3) For minor formations only two alphabets should be used the capital Roman to denote craters, depressions, and dark areas; the small Greek to denote peaks and bright spots.

(4) Such rills as are in Schmidt's catalogue might be denoted by their catalogue numbers, others by a short reference to their position.

I have always thought that the best method of publishing the decisions of the Committee would be to embody them in an accurate map, which must be specially prepared. But the practical difficulties appeared so great that I suggested the use of photographs, very much as Professor W. H. Pickering now does, though I had not thought out the arrangement in so much detail.

Dr. Franz has, however, in a recent letter, suggested the construction of an atlas giving accurate outlines and positions of all the more prominent features, but not presenting the minute detail. The different parts of the surface would be represented in a series of maps, and he has offered to be responsible for the limb regions if I would undertake the central parts. Dr. Franz has made a special study of the limb, and has accumulated a large number of unpublished measures. His maps would be constructed under favourable librations of 10°. I have a large number of measures of the central regions besides those already published; and as Mr. W. H. Wesley has kindly placed his great skill as a draughtsman at the disposal of the Committee for the delineation of these parts, it seems to me that the construction of such a map is now quite feasible. The maps of the central regions would be under mean libration.

I believe that with the data now available a map might be constructed in which the positions of all objects shown would be accurate to within less than 1" of arc (geocentric), corresponding to less than one mile on the surface of the Moon for the central part, whilst the measured points are so thickly distributed that an object not represented in the map, but whose position could be located by cross bearings from points that are represented, could have its co-ordinates read off to very nearly this degree of

accuracy.

With such a map it would be possible to refer to unnamed formations by means of their selenographical co-ordinates, and I believe this to be the best solution of the problem that has yet been suggested.

S. A. SAUNDER.