c plate. It is given here in order
to show that the diameter of the visible disc of the moon does not
diminish as it rises. The slight increase in the breadth of the track
registered by the moon's disc is probably due to a little distortion
caused by the side portion of the lens. After M. Flammarion. The
actual width of the moon's disc as printed here is a little over one
eighth of an inch, which, if we regard it as "a picture" and not
merely as a mechanical record, implies that the observer's eye is only
about 14-1/2 inches distant from the picture plane instead of the more
usual 18 inches, which corresponds to a diameter of the pictured
moon's disc of between 1/6th and 1/7th of an inch (.156 inch).]

If we put a piece of glass coated with a thin layer of water-colour
paint into a frame, and then make a peep-hole in a board which we fix
upright between us and the upright piece of framed glass, we can keep
the framed glass steady (let us suppose it to be part of the window of
a room), and then we can move the peep-hole board back from it into
the room to measured distances. At a distance of one and a half feet
from the framed glass, which is that at which an artist usually has
his eye from his canvas or paper, we can trace on the smeared or
tinted piece of glass the outlines of things seen through it exactly
as they fill up the area of the glass--men, houses, trees, the moon.
The moon's disc (and the same is true of the sun) is found always to
occupy a space on the glass which is 1/115th of the distance of the
eye from the framed glass plate. When the eye-to-frame distance is
eighteen inches, the diameter of the disc of the moon on the smeared
glass will occupy exactly 1/115th of eighteen inches, which is between
one-sixth and one-seventh of an inch. Similarly if the peep-hole is at
nine and a half feet or 114 inches from the framed glass (which stands
for us as the equivalent of an artist's picture) the moon will occupy
almost exactly one inch in diameter--the size of a halfpenny. With
such a simple apparatus of peep-hole and smeared glass in an upright
frame, it is easy to mark off the size covered by the moon (or sun),
whether low or high, on the smeared glass, and it is found never to
vary whether high or low--so long as the same "eye-to-frame" or
"peep-hole" distance is preserved. That seems to be an important fact
for painters of sun-sets and moon-rises. But what do they do? They
never give the right size (namely