At night the cobwebs of the telescope are invisible for want of light; we have to illuminate either the field or the wires, so as to make them visible by contrast. At noon the astronomer meets with the same kind of difficulty from an opposite cause; the intense sun's light has to be obscured by a dark glass, which, at the same time, completely obliterates the spider-lines; these are only seen on the sun's face. In consequenoe, the advent of the sun's edge to a wire cannot be observed ; the line must be fairly on the sun's face before we can see it, and thus the noted instant is necessarily too late,—too late by a quantity depending on the power of the telescope and on the skill of the observer. Hence the estimate of the sun's apparent diameter from observations of the meridian passage may be expected to err slightly in defect, while the thence-deduced right ascension must be too great.