A Description of Helioscopes, and Some Other Instruments

When the brightness and radiance of the Moon, Venus, or Jupiter somewhat bother the eye, they will immediately lose their "beards" original: "beards"; the flares or rays of light that appear to surround bright celestial objects and look very distinct if a single reflection from glass is used in the Telescope.

Another instrument I promised to describe is for measuring diameters, transits, or distances with a certainty of one second original: "second Minute"; a unit of angular measurement equal to 1/60th of a minute, or 1/3600th of a degree. By this means, more may be achieved in finding the parallax The apparent displacement of an object when viewed from two different points, used to calculate distance of the outer planets and the longitude on Earth than has ever yet been done by all the instruments used in the world.

1. This is made exactly, in all particulars, like the Quadrant An instrument used to measure angles up to 90 degrees, regarding its hollow center, threaded limb, screw-frame, and the long rod used to turn the screw from the center. To ensure the screw-frame is kept down more accurately upon the edge of the limb, a small arm should be made to clamp behind the inner limb of the instrument, in the manner represented in Figure 8 by the letter w. By this means, the screw will be kept close, steady, and even against the outward edge of the limb. The letters in this Figure 8 are the same as those in Figures 1 and 11 of the Animadversions Hooke’s 1674 work, Animadversions on the First Part of the Machina Coelestis, where he criticized Hevelius’s use of open sights and represent the same parts, needing no further explanation.

2. Instead of this screw upon a circular limb, a screw may be made to move upon a straight limb or ruler. The end of this ruler must move upon centers or rollers; the centers or axes of these rollers must be exactly in the same line when both the telescopic sights original: "Perspective-fights" are adjusted to the same object and the divisions are begun. The same thing may be done by a straight screw in the manner of a pair of dividing compasses. In that case, the same care must be taken that the axes of the rollers are exactly in the same line. The sides of the encompassing screw, being made of steel, must be made to spring around the long straight screw. This long screw must be made of steel and be at least half an inch in diameter if it is 18 inches long. It will be best to thread it with a fine thread; otherwise, it will be prone to being bent out of a straight line by the force of cutting a large thread. The thread, whether large or small, must be made gradually with a pair of cutting-dies original: "cutting-ftocks" that can be set closer every time the screw is turned.