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

ASTROSCOPIA COMPENDIARIA. 11

or let another hold the lantern nearby. Then, by gradually withdrawing and extending the thread, one should observe whether a double image of the flame appears near the center of the lens, reflected from each of its surfaces. If this occurs when the full length of the thread has been played out—which is, of course, the length required for the future telescope—it is an indication that the lens is directed toward the eye as correctly as possible. But if only one reflection of the flame is seen, it is badly positioned; if neither, it is even worse. Here, however, the remedy will be applied once it is known in which direction the lens declines. For the brass stylus attached to the tip of the tail, which holds the threaded line, must be bent slightly in that same direction; then, as before, the reflection of the lantern must be tested again. This should be done repeatedly until both images of the flame are seen to converge into one. One should use a moderate tension of the thread, such as we defined above, representing the force of two or three pounds, and one should become accustomed to it as much as possible. Once the position of the lens is corrected in this way, it will suffice for all observations. Nor should anyone object too subtly here that, due to the oblique rise of the thread when it is directed toward the stars, its deflection caused by gravity is slightly less than when the same thread is extended parallel to the horizon. This difference is, in fact, very slight, especially given the great lightness of the thread; and the parallelism of the lenses, as we have already said, is not required to be exact to the laws of geometry.

The wind would have to be said to be much more harmful, as it causes the thread to sway and pushes it to the side, especially at the great lengths we have mentioned. Except that the same wind is also adverse to tubes, which tremble and waver from its vibration to the great inconvenience of the observer, such that for this reason, one must often forgo observations. But so that we may bear these losses with a more even mind, it should be known that when the winds blow, the clarity of the air is almost always so disturbed, even if it appears serene, that this alone hinders the entire view of the telescopes. This cannot be unknown to the experienced. Indeed, even in a calm and completely clear sky, when the stars are twinkling most brightly, telescopes are nevertheless used in vain. A certain moist vapor obscures the air, which causes a certain trembling and fluctuating undulation to intercept the entire sharpness of vision for one looking at the bodies of the Planets. And where this happens, the quality of the lenses themselves could be suspected,