CHRISTIAAN HUYGENS

A geometric diagram depicts a series of intersecting lines and segments, labeled with lowercase letters g, h, l, a, b, c, e, and f, illustrating the curvature of the thread and the resulting angle of deflection.

ce, af, parallel to db. Therefore, to the observer from point a, along the line ae, it was noted that the space ce becomes one foot; from which db is the fourth part of a foot. But af is equal to ce. Thus, the thread cba pulls the lens placed at c in such a way that it is not directly opposite the eye, which is at a, but to point f. So that the eye is at an interval of one foot from the true location: which in that distance of 150 feet cannot be harmful. For the angle of deflection cae or acf becomes only two-fifths of one degree; so that there is no need for the remedy, which we shall nevertheless give. But with distance gh taken as double the former, or three hundred feet, so that the thread gbh is curved, the measure of the cavity kb will indeed be quadruple the former db, but the angle of deflection only double, that is, one degree; as is easily perceived, by drawing the tangent gl, which meets the perpendicular hl. For hl itself will be quadruple to kb or ce; but the distance gh to ac was double. Therefore, the angle of deflection hgl can be considered double that of the cae previously found.

This aberration, however, of 48 minutes is still of no importance, nor will it be harmful if neglected. Nevertheless, so that there remains no room for complaint here, I will now show what kind of correction can be applied, and indeed one such that, in one operation, it restores every other declination of the lens.

Therefore, once from the beginning, for the superior preparation of the large lens, let this which we shall say be added. Namely, with the lens balanced as we have instructed, and fixed at the height of the eye, let the thread attached to the tail be taken with the other hand, and brought to the eye...