Of these methods, the simplest on the whole seems to be that given by Mr. W. F. Denning in his admirable volume, 'Telescopic Work for Starlight Evenings.' Fasten on the diaphragm of an eye-piece (the blackened brass disc with a central hole which lies between the field and eye lenses of the eye-piece) a pair of fine wires at right angles to one another. Bring the edge of the sun up to the vertical wire, the eye-piece being so adjusted that the sun's motion is along the line of the horizontal wire. This can easily be accomplished by turning the eye-piece round until the solar motion follows the line of the wire. Then note the number of seconds which the whole disc of the sun takes to cross the vertical wire. Note, in the second place, the time which the spot to be measured takes to cross the vertical wire; and, having these two numbers, a simple rule of three sum enables the diameter of the spot to be roughly ascertained. For the sun's diameter, 866,000 miles, is known, and the proportion which it bears to the number of seconds which it takes to cross the wire will be the same as that borne by the spot to its time of transit. Thus, to take Mr. Denning's example, if the sun takes 133 seconds to cross the wire, and the spot takes 6·5, then 133: 866,000::6·5:42,323, which latter number will be, roughly speaking, the diameter of the spot in miles. This, method is only a very rough approximation; still, it at least enables the observer to form some conception of the scale of what is being seen. It will answer best when the sun is almost south, and is, of course, less and less accurate as the spot in question is removed from the centre of the disc; for the sun being a sphere, and not a flat surface, foreshortening comes largely and increasingly into play as spots near the edge (or limb) of the disc.