6289. OPTICS, Laws of.—
To distinct vision
it is necessary not only that the visual
angle should be sufficient for the powers of the
human eye, but that there should be sufficient
light also on the object of observation. In
microscopic observations, the enlargement of
the angle of vision may be more indulged,
because auxiliary light may be concentrated
on the object by concave mirrors. But in the
case of the heavenly bodies we can have no
such aid. The moon, for example, receives
from the sun but a fixed quality of light. In
proportion as you magnify her surface, you
spread that fixed quantity over a greater space,
dilute it more, and render the object more dim.
If you increase her magnitude infinitely, you
dim her face infinitely also, and she becomes
invisible. When under total eclipse, all the
direct rays of the sun being intercepted, she
is seen but faintly, and would not be seen at
all but for the refraction of the solar rays in
their passage through our atmosphere. In a
night of extreme darkness, a house or a mountain
is not seen, as not having light enough to
impress the limited sensibility of our eye. I do
suppose in fact that Herschel has availed himself
of the properties of the parabolic mirror to
the point beyond which its effect would be
countervailed by the diminution of light on the
object. I barely suggest this element, not presented
to view in your letter, as one which must
enter into the estimate of the improved telescope
you propose.—
To Thomas Skidman. Washington ed. vii, 259.
(M.
1822)
