Christiani Hugenii Const. F. Astroscopia Compendiaria, Tubi Optici molimine liberata

CHRISTIANI HUGENII 12

unless it had been known at another time and under a purer sky. The same vapor, let me also mention this in passing, not infrequently adhering to the larger lens, averts a part of the light rays. This evil is countered by warming the glass moderately near a fire.

Let us now see also that which we said about illuminating the same lens when it is raised on the mast. If it is very far away, say at an interval of two hundred feet or more, it hardly seems that it will receive enough light to be perceived by the observer, even if the lantern is assisted by a convex glass, as we have instructed. But here it will be permitted to increase the light further, either by enlarging the wick of the lantern itself, or by using a wider and more gently convex lens, which, even if it receives the transmitted light in equal quantity, will nevertheless diffuse it less, and consequently will project it further.

As far as these matters are concerned, it is clear that it matters not at all what the length of the telescope has been, but that any length can be just as easily put to use. The only difference lies in the varying height of the mast. Several methods are available for preparing this. For we can, with one mast established, raise another twice as high alongside it by its means, and at the same time make it firmer by joining both together with transverse slats. And such a structure will be very firm if two lower masts, along with a third of double height, are set two or three feet apart from each other, arranged in a triangle, and bound together as we have said. In this way, we will easily reach a height of one hundred feet. For much greater heights, however, either by using a stronger substructure of masts and beams, or by attaching the lower timbers to a tower or the corner of a higher building, so that nothing prevents the primary lens from being moved from the bottom to the top, ascending through a continuous small channel as we have said. But a mast can also be erected on top of a tower or the roof of a house, so that he to whom the care of the rope has been entrusted may stand there to raise or lower the lens.

Nor should anyone think that this care is addressed by us prematurely or superfluously, because it is not considered likely that these heights will be needed.