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.