where the Sun's Image will be formed in a fmall round Spot. If then, by means of the Screw the small Mirrour gh ́be moved to a little more than its focal Distance from the Image of the Sun at f, the Rays which go from it to the Speculum gh, will by it be reflected through the Hole HO in the large one, to a plano-convex Glafs behind at IK, which will cause a second and much larger Image to be formed in the Part L M in the Tube behind; this fecond Image is viewed by a Plano-convex Eye-glafs, placed in the Part NQ by parallel Rays paffing thro' a fmall Hole to the Eye at R. 58. But as this is a very complex and intricate Calculation, I must refer the Reader to my Philofophia Britannica for the Method of computing the magnifying Power of this Inftrument, and fhall here only observe, that it is fo great that a reflecting Telescope, whose focal Diftance of the great Mirrour is any Number of Inches, will generally magnify as much as a refracting Telescope which is as many Feet in Length. I might also here obferve, that this Inftrument is as well adapted for a Microscope as a Telescope, but this is beft fhewn by Experiment, and that in different Way's *. 59. There is a Method of applying either the refracting or reflecting Telescope in a *See more at large the manifold Ufes of a Reflecting Telescope in my OPTICAL ESSAYS, lately published. dark dark Room for viewing the Spots in the Sun, the Eclipfes of the Sun, the beautiful Colours of the Clouds, &c. &c. But for this Purpose the Refracter is beft, and may be properly. called the Solar Telescope. This likewise we fhew in all these Cafes by Experiments in the Camera Obfcura. SECTION III. The Nature and Properties of AIR explained, the artificial Production of AIR, of the TRANSPARENCY, ELASTICITY, and GRAVITY of the Air, of BAROMETERS, THERMOMETERS, and HYGROMETERS, the Conftruction and Ufe of a NEW PORTABLE AIR-PUMP, a felect Number of principal Experiments thereon, to illuftrate the Qualities and Ufes of the Air. I. HE Bufinefs of this Section will be THE difcourfe of the Nature, Origin, and Properties of AIR, and the Defcription and Ufe of thofe Inftruments and Machines by which they are demonftrated in a Course of Experiments, according to the modern Improvements and Discoveries. 2. The Nature of AIR confifts in the following Particulars: (1) Extreme Subtilty or Smallness of its Particles. (2) The Inifibility of its Parts, and the Transparency thence arifing. (3) Its Springiness or Ela fticity. |