1875.]  Selenography : tts Past, Present, and Future. 207 
representations of the real condition of the lunar surface, 
enabling its details to be examined with ease at leisure, they 
have hitherto proved entirely inadequate for this purpose, 
even the finest photographs exhibiting hardly a trace of this 
detail, but show only the coarser features of the larger for- 
mations and the mere existence of the smaller, whilst even 
what is to be seen appears at times, from differences in 
actinic and illuminating power, so different to its actual 
appearance as to render it recognisable only with difficulty. 
Those already obtained seem to be deficient in sensitiveness 
to slight variations in illumination, which prevents the 
smaller detail being separately distinguishable, but blurs the 
whole into one general mass, which no mere enlargement 
can rectify, and it is doubtful whether this fault can be over- 
come, except by taking a larger image directly, even if 
that proves successful. Whatever may be the cause, . 
the result has been to render lunar photographs of little 
assistance to the study of the details of the surface of the 
moon, as Madler has already pointed out. 
The other respect in which much was anticipated from lunar 
photographs was in the accurate mapping of the surface of the 
moon, as it was considered they would afford a means by 
which the true positions of the lunar formations might be 
laid down by means of a series of measures. In this, how- 
ever, they have hitherto been of little service, only having 
been employed by Birt to lay down secondary positions on 
the lunar map of the British Association, by whom they 
were also employed to give the approximate dimensions of 
the principal formations. For these purposes, were the 
number of well-determined points much increased, they 
might prove of considerable assistance, and more use made 
of them for this end than has been. But for the determi- 
nation of points of the first order with any accuracy, they 
are far inferior to direct measures, and the results neither 
satisfactory nor sufficiently trustworthy perhaps. Even 
were there no uncertainty introduced by the want of sharp 
definition, or by variations of illumination, all certainty is 
lost in the errors that can be, or perhaps must be, introduced, 
when dealing with such delicate material as collodion film 
and paper. And it appears impossible to guard against the 
effect of the vagaries of such materials as collodion films to 
the desired amount of accuracy, and the use of daguerreo- 
types seems impracticable, considering the large scale 
necessary. 
With regard to classes of measured points on the lunar 
surface, on which depend our power of constructing a 
