He made two small ones, with an Object metal spherically concave, the second being better than the other; the word of which he describes in the Philosophical Transactions, (at Numb. 80.) and the other he sent to the Royal Society. This worst was not long ago to be seen at Mr Heath’s, the Mathematical Instrument Maker in the Strand, having upon it, wrote with his own Hand, Isaac Newton.
AB (Fig. 20.) is the concave Speculum, of which the Radius or Semidiameter is 12
The Tube of this Telescope is open at the End which respects the Object, the other End is close, where the said Concave is laid. The flat oval Speculum is near the open End, made as small as may be, the less to obstruct the Entrance of the Rays of Light, where is a little Hole furnished with the said small plano-convex Eye-Glass. So that the Rays coming from the Object do first fall on the Concave placed at the Bottom of the Tube, and are thence reflected towards the other End of it, where they meet with the flat Speculum obliquely posited, by the Reflection of which they are directed to the little plano-convex Glass; and so to the Spectator’s Eye, who looking downward sees the Object which the Telescope is turned to.
This Telescope will magnify as to Surface about 38 Times, viz. as much as a refracting Telescope of 2 Feet, and by it you may read in the Philosophical Transactions, at the Distance of too or 120 Feet.
Sir Isaac Newton, in his Opticks, page 95, gives another Description of a Reflecting Telescope, which see there.
A little after Sir Isaac Newton had sent his Telescope to the Royal Society, Mr Oldenburgh, the Secretary, wrote him a Letter of Thanks, to which Sir Isaac Newton made Answer in 167, giving a farther Account of the Instrument.
About this Time Dr James Gregory having an Account of Sir Isaac Newton’s Telescope, wrote his Thoughts about it to Mr Collins, in a Letter from Aberdeen, dated August 6. 1672. In which he gives the Preference to Sir Isaac’s Telescope, above that which he described in the Optica Promota, in one respect, but thinks his own better in another. At the same Time one Mr Cassegraine, a French Man, published a Description of a Catadioptrick Telescope, as his own Invention, which he pretended had been prior to Sir Isaac’s Telescope. Sir Isaac contrived his in the Year 1666, and executed it in the Year 1670, or 1671. And indeed Dr James Gregory his before either of them, viz. before 1663. This Telescope of Mr Cassegraine’s differs nothing from the Doctor’s, excepting that he would have the small Metal to be convex, whereas in that of the other it is concave; but I never heard that an Instrument of this Kind was ever yet made. See an Account of it, in the Philosophical Transactions, at Numb. 83. In the second Volume of Dr Smith’s Opticks, amongst the Remarks, page 105. he says, that what Trials have been made of Mr Cassegraine’s Form he knows not; but it appears by a Table there setdown, for the Parts of this Telescope, compared to another there let down for the Parts of Gregory’s Telescope, that the former has the Advantage, being shorter by twice the focal Distance of the lesser Speculum, and yet magnifies more.
Sir Isaac Newton says, the Telescope he made, was but 6 Inches long, bears something more than an Inch Aperture, with a plano-convex Eye-Glass, which is