A third method--to measure the mountain profile directly by means of the micrometer--is evidently applicable only to altitudes lying exactly on the lunar rim." "That is clear enough," said Ardan, "and another point is also very clear.

In Full Moon no measurement is possible.

When no shadows are made, none can be measured.

Measurements, right or wrong, are possible only when the solar rays strike the Moon's surface obliquely with regard to the observer.

Am I right, Signor Barbicani, maestro illustrissimo ?" "Perfectly right," replied Barbican.