ON MAPPING THE SURFACE OF THE MOON. 245 



Catalogue. 



In the following Catalogue every feature seen on the photographs (as well 

 as some of the more minute detail discoverable by the telescope) is described. 

 The descriptions include all the points of interest that have presented 

 themselves in the examination of this part of the moon's surface, either by 

 the aid of photographs or by means of personal observation ; and some at- 

 tempt has been made to assist in obtaining a more correct representation of 

 the moon's surface, by giving the measures in seconds of arc of the objects 

 catalogued, and in the case of craters the deduced apparent magnitudes, the 

 crater Dionysius being considered as the standard. These measures and de- 

 duced magnitudes can only be regarded as roughly aj^prosimate, the epochs 

 being mean distance and mean libration ; they cannot consequently be ex- 

 pected to coincide with any actual measures ; for as the epoch of mean libra- 

 tion cannot be coincident with that of mean distance, measures made at mean 

 distance wiU not agree with those made at mean libration. The basis of 

 measurement is as follows : — the semidiameter of the moon 15' 32"*27, as 

 given by Oudemans from occultations and direct measurements by the helio- 

 meter (see Monthly Notices of the Royal Astronomical Society, vol. xxvi. 

 p. 260), has been adopted, and as this gives lS64"-54 for the diameter, a 

 scale has been adapted to the datum 100 inchcs=18G4"-54. Eutherford's 

 photograph, from which the measurements have been taken, is 20-875 inches 

 in diameter, and as the measures are readily convertible from 20-875 to 

 100 inches, thej^ are at once referred to mean distance, irrespective of the 

 actual position of the moon, either as to distance or libration, and, as before 

 stated, can only be considered as approximate ; nevertheless it is hoped they 

 may be of some service both in obtaining a better acquaintance with the moon's 

 surface, and also contributing to its being more accurately represented. 



The magnitudes /or the epoch of the photograph have been deduced as de- 

 scribed in my last Eeport (Eeport, 1865, p. 295), but the values are pro- 

 bably too small, as the measured diameter on the photograph of Dionysius 

 appears to be excessive, the crater being surrounded by a fringe of light*. 

 As most of the measures for magnitude have been made in a direction at 

 right angles to a line joining the north edge of Dionysius and the south edge 

 of Agrijjpa, the deduced magnitudes may be easily corrected at any futui-e 

 time by measurements in the same direction. 



It is proper to remark that the positions have been laid down and the 

 magnitudes in the first instance determined by a scale, radius =50 inches, all 

 measures being expressed in parts of this scale, radius=umty. For the 

 expression of aU quantities in seconds of arc the measurements have been 

 re-made, and directly referred to the scale, 100inches=1864"-54, quite irre- 

 spective of the scale 50 inches = radius. The magnitudes have been deter- 

 mined on both scales, and the resulting mean adopted ; some discrepancies, 

 however, will be found between the mcasiircs in seconds of arc and the 

 magnitudes, which, as before stated, are the resiilts of two sets of measures, 

 while the value in seconds of arc depend on one only. The greatest number 

 of differences in this respect occur amongst those objects that range between 

 3" and 4", but as they do not materially interfere with the gradations of 

 magnitude which range between 0'"-00 and 0"-70, it is considered best to 



* This fringe of light appears to vary in size, the differences in above 70 sets of mea- 

 sures of the diameter of Dionysius in various directions inchidcd hy an angle of 50" 

 amounting to about G" or 4" more than can be accounted for by the varying distances of 

 the moon. 



