216 Selenography : its Past, Present, and Future. [April, 
careful investigation, a work necessarily of considerable 
labour. Moreover, it appears certain that the position of 
numbers of the formation will need alteration, when their 
true places have been ascertained by systematic measures. 
Being founded likewise on drawings made at different 
epochs, some uncertainty must attach to the combination 
of these into a single connected whole, whilst, as is known, 
a considerable personality attaches to lunar observation, 
one observer detecting certain classes of objects far readier 
than others. Much also remains to be done in the proper 
combination of the isolated details seen at different times 
into a connected whole, and here appears one of the most 
difficult selenographical labours. For on the moon, the 
details of the surface being nearly only rendered visible 
when thrown in relief by shadow, it is rarely that an entire 
formation is seen equally distin¢ét at the same time, but 
neatly always the details come out little by little, those 
first seen disappearing as the newer come into view; so that 
it is necessary, from isolated observations, to piece together, 
as) it were, thevdetails): into a connected whole.  Con- 
sidering the great difficulty in studying these details, and 
the fact that much is only visible under conditions when the 
rest is quite invisible, while some are rarely, if ever, seen at 
all, the great labour and difficulty in piecing all together so 
as to obtain the nature of the formation which conne¢ts 
together all the isolated detail seen is apparent; whilst it 
must be remembered that, owing to the effects of libration, 
the appearance of these details are constantly altering. 
Lohrmann, and Beer and Madler, have seldom attempted to 
do this, but have merely mapped down the object seen; and 
a similar course seems to have been followed in the main 
by Schmidt; but the study of the real nature of the surface 
will necessitate its being carried out in full, if we are ever 
to properiy comprehend the meaning of what is visible on 
our satellite. 
So far no reference has heen made to the results obtained 
by the employment of photography in selenographical in- 
vestigation, though many fine photographs of the moon 
have been obtained mainly through the labours of De la Rue 
and Rutherfurd. Hitherto, however, the results obtained 
through the aid of photography have been of less value 
than was generally anticipated, for though for many pur- 
poses the photographs have been and will continue to be of 
great service, with regard to the two principal ends, it was 
trusted they would be of the greatest value: they have failed 
to realise expectation. In so far from affording unerring 
