Insert 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.