Humboldt enumerates the following different methods adopted for the
Measurement of Light: a comparison of the shadows of artificial lights,
differing in numbers and distance; diaphragms; plane-glasses of
different thickness and colour; artificial stars formed by reflection
on glass spheres; the juxtaposition of two seven-feet telescopes,
separated by a distance which the observer could pass in about a
second; reflecting instruments in which two stars can be simultaneously
seen and compared, when the telescope has been so adjusted that the
star gives two images of like intensity; an apparatus having (in
front of the object-glass) a mirror and diaphragms, whose rotation
is measured on a ring; telescopes with divided object-glasses, on
either half of which the stellar light is received through a prism;
astrometers, in which a prism reflects the image of the moon or
Jupiter, and concentrates it through a lens at different distances into
a star more or less bright.--_Cosmos_, vol. iii.