A photograph of the sun taken, it has next to be measured, the four following particulars being determined for each spot: First, its distance from the centre of the image of the sun; next, the angle between it and the north point; thirdly, the size of the spot; and fourthly, the size of the umbra of the spot, that is to say, of its dark central portion. The size or area of the spot is measured by placing a thin piece of glass, on which a number of cross-lines have been ruled one-hundredth of an inch apart, in contact with the photograph. These cross-lines make up a number of small squares, each the ten-thousandth (1/10000 in.) part of a square inch in area. When the photograph and the little engraved glass plate are nearly in contact, the photograph is examined with a magnifying glass, and the number of little squares covered by a given spot are counted. It will give some idea of the vast scale of the sun when it is stated that a tiny spot, so small that it only just covers one of these little squares, and which is only one-millionth of the visible hemisphere of the sun in area, yet covers in actual extent considerably more than one million of square miles. The dark spots are not the only objects on the sun's surface. Here and there, and especially near the edge of the sun, are bright marks, generally in long branching lines, so bright as to appear bright even against the dazzling background of the sun itself. These are called 'faculæ,' and they, like the spots, have their times of great abundance and of scarcity, changing on the whole at the same time as the spots. After the solar photographs have been measured, the measures must be 'reduced,' and the positions of the spots as expressed in longitude and latitude on the sun computed. There is no difficulty in doing this, for the position of the sun's equator and poles have long been known approximately, the sun revolving on its axis in a little more than twenty-five days, and carrying of course the spots and faculæ round with him.