Aristotle maintains that Stars may occasionally be seen in the
Daylight, from caverns and cisterns, as through tubes. Pliny alludes
to the same circumstance, and mentions that stars have been most
distinctly recognised during solar eclipses. Sir John Herschel has
heard it stated by a celebrated optician, that his attention was first
drawn to astronomy by the regular appearance, at a certain hour, for
several successive days, of a considerable star through the shaft of
a chimney. The chimney-sweepers who have been questioned upon this
subject agree tolerably well in stating that “they have never seen
stars by day, but that when observed at night through deep shafts,
the sky appeared quite near, and the stars larger.” Saussure states
that stars have been seen with the naked eye in broad daylight, on
the declivity of Mont Blanc, at an elevation of 12,757 feet, as he
was assured by several of the alpine guides. The observer must be
placed entirely in the shade, and have a thick and massive shade above
his head, else the stronger light of the air will disperse the faint
image of the stars; these conditions resembling those presented by the
cisterns of the ancients, and the chimneys above referred to. Humboldt,
however, questions the accuracy of these evidences, adding that in the
Cordilleras of Mexico, Quito, and Peru, at elevations of 15,000 or
16,000 feet above the sea-level, he never could distinguish stars by
daylight. Yet, under the ethereally pure sky of Cumana, in the plains
near the sea-shore, Humboldt has frequently been able, after observing
an eclipse of Jupiter’s satellites, to find the planet again with the
naked eye, and has most distinctly seen it when the sun’s disc was from
18° to 20° above the horizon.