Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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I Afterwards Employed Diaphragms
Diminishing The Aperture Of The Telescope, And Coloured And
Colourless Glasses Placed Before The Eye-Glass.
I moreover made use
of an instrument of reflexion calculated to bring simultaneously
two stars into the field of the telescope, after having equalized
their light by receiving it with more or fewer rays at pleasure,
reflected by the silvered part of the mirror.
I admit that these
photometric processes are not very precise; but I believe the last,
which perhaps had never before been employed, might he rendered
nearly exact, by adding a scale of equal parts to the moveable
frame of the telescope of the sextant. It was by taking the mean of
a great number of valuations, that I saw the relative intensity of
the light of the great stars decrease in the following manner:
Sirius, Canopus, a Centauri, Acherner, b Centauri, Fomalhaut,
Rigel, Procyon, Betelgueuse, e of the Great Dog, d of the Great
Dog, a of the Crane, a of the Peacock. These experiments will
become more interesting when travellers shall have determined anew,
at intervals of forty or fifty years, some of those changes which
the celestial bodies seem to undergo, either at their surface or
with respect to their distances from our planetary system.
After having made astronomical observations with the same
instruments, in our northern climates and in the torrid zone, we
are surprised at the effect produced in the latter (by the
transparency of the air, and the less extinction of light), on the
clearness with which the double stars, the satellites of Jupiter,
or certain nebulae, present themselves.
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