ert this between the anterior and posterior lenses
of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
telescope upon the sun, and having so arranged it that the whole disc of
the sun may be projected on the screen, count carefully the number of
graduations that are seen to exactly occupy the solar diameter.... It
matters not in which direction you measure your diameter, provided only
the sun has risen some 18 deg. or 20 deg. above the horizon, and so escaped the
distortion occasioned by refraction.[16]

"Next let us suppose that our observer has been observing the sun on any
day of the year, say, if you choose, at the time of its mean apparent
diameter, namely about the first of April or first of October, and has
ascertained that" (as is the case with Mr. Howlett's instrument)
"sixty-four graduations occupy the diameter of the projected image. Now
the semi-diameter of the sun, at the epochs above mentioned, according
to the tables given for every day of the year in the 'Nautical Almanac'
(the same as in Dietrichsen and Hannay's very useful compilation) is
16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
now we divide 1924" by 64" this will, of course, award as nearly as
possible 30" as the value in celestial arc of each graduation, either as
seen on the screen, or as applied directly to the sun or any heavenly
body large enough to be measured by it."

Since the sun's diameter is about 850,000 miles, each graduation (in the
case above specified) corresponds to one-64th part of 850,000
miles--that is, to a length of 13,256 miles on the sun's surface. Any
other case can be treated in precisely the same manner.

It will be found easy so to place the screen that the distance between
successive graduations (as seen projected upon the screen) may
correspond to any desired unit of linear measurement--say an inch. Then
if the observer use transparent tracing-paper ruled with faint lines
forming squares half-an-inch in size, he can comfortably copy directly
from the screen any solar phenomena he may be struck with. A variety of
methods of drawing will suggest themselves. Mr. Howlett, in the paper I
have quoted from above, describes a very satisfactory method, which
those who are anxious to devote themselves seriously to solar
observation will do well to study.

It is necessary that the observer should be able to determine
approximately where the sun's equator is situated at the time o