16 REPORT— 1869. 



of this being actually the case. The scales of tint and brilliancy adopted by the 

 three leading selenographers of the present century differ from each other, and the 

 observations which have been referred to solar altitudes and azimuths are so ex- 

 ceedingly few, that legitintatebj to connect variations of brilliancy with change of 

 illuminating angle is quite out of the question ; still the presumption is strong that 

 the most striking changes of tint and brilliancy are connected with variations of 

 illuminating angle. It is not, however, light alone that affects the objects in the 

 way observed ; tlie nature of the materials of which they are composed plays an 

 important part, some reflecting more, some less light than others. 



When we enter upon a comparison of tints, eitlier observed directly by means of 

 the telescope or recorded on photogranis, with those given on our maps or recorded 

 in the works of selenographers, we are often struck with the differences thus de- 

 tected ; but as we fail to connect by ordinary inductive processes apparent vari- 

 ation of brightness with change of illuminating angle, simply on account of the 

 absence, on the one hand, of a suitable scale of brightness, and on the other, of a 

 discussion of the observations with respect to solar altitudes and azimuths, so with 

 regard to the differences just alluded to, we fail to refer them to change of a phy- 

 sical character, just because we are destitute of the necessary evidence that the 

 spots were really darker or lighter than they are at present. 



While the scales of SchriJter, Lohrmann, and Beer and Miidler differ among 

 themselves, the tints or brilliancy ascribed by each selenographer to the objects 

 recorded by him are comparable one with the other upon his particular scale, and 

 thus a means exists of ascertaining at the epoch of each whether one of any two 

 spots was brighter or darker than the other at the phase at which the brilliancy 

 was determined. It is obvious that now, observations of the brightness of the same 

 two spots may be obtained at about tlie same phase, provided the suitable oppor- 

 tunities be embraced, and thus a change of brilliancy or tint may be detected of 

 a different character to that dependent on change of illuminating angle ; for if the 

 observations be made at precisely the same phase as when the brightness was re- 

 corded, it is clear that the illuminating angle must be nearly if not quite the same, 

 consequently the variation of tint or brightness in one or both of the two spots must 

 be referred to some agency different from illumination. 



Last year I solicited the attention of the Section to the difference of tint which 

 characterized a somewhat large shallow crater near Alpetragius, as compared with 

 the drawings of Lohrmann, Beer and Miidler, and Schmidt. The floor of this crater 

 was seen to be darker in 1808 than the siu-rounding surface, and therefore darker 

 than delineated by the selenographers just named. The same dark tint, imder every 

 phase at which it has been examined, has been observed without exception since 

 August 18G8. The legitimate conclusion is, that during the period of the obser- 

 vations in 1868 and in 1869, the surface of this crater was permanently the darkest 

 in the neighbourhood. If Lohrmann, Beer and Miidler, and Schmidt were correct 

 in their comparative delineation of the tint of this crater (and it is difficult to con- 

 ceive that three experienced selenographers could have fallen hito the same error), 

 we have presumptive evidence of a phenomenon which may be termed " a secular 

 variation of tint." What it arises from is altogether another question ; it, however, 

 does not stand alone on the sm-face of the moon. 



To enter into any speculation as to the cause of such variations, whether from 

 bright to dark, or the reverse, is manifestly premature ; there is, however, a class 

 of phenomena which bears much on the same point ; the surfaces of many of the 

 larger smooth-walled plains are greatly diversified with spots, streaks, and in some 

 cases spreading fans of light ; the rays issuing from Proclus and spreading over the 

 Mare Crisium may be cited as examples of the latter. The walled plain Plato has 

 been the subject of numerous obsen-ations, both as regards its very interesting 

 mountain border and still more interesting floor. No two drawings of this floor 

 that I have seen precisely agi-ee, a result to be expected when we consider that 

 the differences of illuminating and visual angle tend materially to influence its as- 

 pect as seen fi'om the earth, added to which, we have the difterent impressions pro- 

 duced at different times on artists and observers ; still the differences dependent 

 upon illumination &c. will have a limit ; certain well-known features will always 

 be recognized, although somewhat altered in appearance at different times j it is 



