8 . REPORT 1864. 



general surface, whether as clouds or mountains, to either of which they may be 

 truly likened. In respect to porosity, the author had also devoted much time to 

 a scrutiny of the interspaces between the faculre towards the limb and the general 

 surface towards the interior of the disk. Towards the interior the ground acquires 

 more evident lights and shades, a sort of granulation difficult to analyze. Under 

 favourable conditions for observation, there appears little or none of that tremor 

 and internal motion described by earlier observers. What is then seen is a com- 

 plicated surface of interrupted lights and shades, the limits of which appear 

 arched, or straight, or confused, according to the case; and the indeterminate 

 union of these produces sometimes faint luminous ridges, the intervals tilled up by 

 shaded interstices and insulated patches of iUmniuated surface. The best resem- 

 blance to these complicated small surfaces of light and shade he had been able to 

 procure was a disk of a particular sort of white paper placed near the eye-end of 

 the telescope, and seen by transmitted light. Heaps of small fi-agments of white 

 substances, not so imiform in figure or equal in size as rice-grains, might also be 

 suggested for comparison. 



On a susjyected Change of Bnglitness in the Lunar Si^ot, Werner. 

 Bij the Rev. T. W. Webb, A.M., F.R.A.S. 

 The mysterious appearance of the Moon under high illumination, and the want of 

 accordance between its actual relief and what may be called its local colourimj, have 

 by no means received a degree of attention corresponding with the present position 

 and requirements of science, or with the unprecedented optical resources now at 

 our command. The investigation would no doubt present many difficulties, but 

 we ought not to be thus deterred from attempting the solution oif so interesting a 

 question ; and a more persevering and minute examination of the topogi'aphy of 

 the full Moon could scarcely fail of meeting with its reward in a well-marked ad- 

 vance in the boimdary of our knowledge. One probable result might be the dis- 

 covery of variation in the brightness of the lummous markings. No reason can be 

 given for acquiescing in the general supposition of their permanence, except the 

 testimony of very inadequate representations. The changes remarked by Schriiter 

 and Gruithuisen are periodical, and therefore of another character ; but a suspicion 

 has begim to be entertained of more permanent alteration. Messrs. Birt and Hunter 

 (the Earl of Rosse's obseiwer), as well as the present wi-iter, have found the interiot 

 oi Plato different fi-om the representation of Beer and Miidler; and the object of 

 this paper is to bring forward evidence of another change, in the interior of the 

 crater Werner. A small luminous spot at the foot of the wall on the N.E. side has 

 been twice referred to by Beer and Madler, in the most distinct and positive man- 

 ner, as equalling the brightness of Aristarchus, and surpassing in this respect every 

 other portion of the lunar disk. Such, according to the observations of the present 

 writer, is no longer the case. In 1855 and 1856 its comparative inferiority was 

 noticed with two achromatics of 3-7 inches aperture ; and during the present year, 

 a careful investigation with a 5'5-inch object-glass imder very various angles of in- 

 cident light, with many magnifying powers, and the occasional emplojanent of 

 coloured screen-glasses, leads to the same conclusion. So far from rivalling the 

 intensity oi Aristarchm, it never equalled that ot Proclus, Censorinus, or Dionydus, 

 all which are rated lower by Beer and Madler. From its position it does not seem 

 likely that other circimistances of libration would influence its apparent brightness ; 

 and since Beer and Madler have studied the vicinity with especial minuteness 

 (much more than they have bestowed on some other regions), it appears highly 

 probable that the spot has decreased in brilliancy during the lapse of twenty years. 

 A careful examination of other districts might probably detect similar changes. 

 For this purpose, in place of vague and arbitrary estimations, a method of sequences 

 similar to that adopted by Su* J. Herschel in stellar photometry might prove of 

 great service, and coloured screen-glasses of various depths would be usefid ; but 

 care must be taken to discriminate between the impressions of extent and intensity, 

 and to avoid fragmentary comparisons with imequal apertures, as it is probable 

 that the decrease of differences in apparent brightness with increasing light, which 

 J8 known to obtain in the case of stars, may find place also in these observations. 



