214 Selenography : its Past, Present, and Future.  [{April, 
occurred, though no explanation can be given of the appear- 
ance as seen by Lohrmann, and Beer and Madler. The 
importance of measures of the principal lunar formations 
was, however, apparent; for had not Lohrmann and Madler 
measured Linne, it is doubtful whether it would have 
received the attention merited on Schmidt’s announcement; 
whilst had they and Schmidt measured carefully the dimen- 
sions of the crater, no uncertainty could have existed as to 
its real appearance. Schmidt and others have since pointed 
out cases of apparent change, but being in unmeasured 
points, have received little elucidation. During the period 
1869-71, many observations were made of the relative 
visibility of the minute crater cones, white spots, and streaks 
on the surface of the walled plain Plato, and the results have 
been discussed under the direction of a British Association 
Committee, whose reports were published in 1871 and 1872, 
where it was shown that the minute objects on Plato under- 
went curious and inexplicable variations in visibility ; whilst 
the whole floor showed an apparent darkening of marked 
extent during each lunation. Numerous observations have 
also been made, indicating peculiar changes in the visibility 
of different lunar regions, not dependent on variation of 
illumination; many new formations have been detected, 
now so conspicuous as to appear hardly capable of being 
overlooked by earlier astronomers, were they then as distin¢t; 
changes in tint and colour have been recognised ; and finally, 
considerable differences have been noticed in the position of 
the light streaks from Madler’s drawings, whilst a number 
of new ones have been seen. ‘The result, in short, of the 
observations of the last period of selenographical activity 
has been to reopen all the questions nearly previously con- 
sidered settled by Beer and Madler, before the true nature 
of the lunar details were generally understood, and with 
regard to which the small aperture of their telescope placed 
them under serious disadvantages. 
The present condition of selenography is essentially, 
therefore, one of uncertainty; and though the earlier astro- 
nomers have by their work cleared the ground, and defined 
the questions to be solved, their solution is still to be accom- 
plished, and for this end further observations are necessary 
to afford the requisite data. 
The principal points to which this further micrometrical 
work should be directed are essentially the following :—The 
determination of an adequate number of lunar positions, 
comprising those of all the principal formations; the mea- 
surement of the dimensions of the principal craters and of 
