Mathematical Instruments
Suppl. Ch. VIII.

# A Description of the Reflecting Telescope, made by Sir Isaac Newton himself.

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$$\frac{2}{3}$$ or 13 Inches. CD is another metalline Speculum, whose Surface is flat, and the Circumference oval. GD is an iron Wire, holding a Ring of Brass, in which the Speculum CD is fixed. F is a small Eye-Glass, flat above, and convex below, of the twelfth Part of an Inch Radius, if not less; forasmuch as the Metal collects the Sun’s Rays at 6$$\frac{1}{3}$$. Inches distance from it’s vertex. GGG the fore-part of the Tube is fastened to a brass Ring HI to keep it immoveable. PQKL is the hind-part of the Tube fastened to another brass Ring PQ. O is an iron Hook fastened to the Ring PQ, and furnished with a Screw N. Thereby to advance or draw Back the hind-part of the Tube, and so by that means to put the Speculums to their due Distance. MQG is a crooked Iron sustaining the Tube, and fastened by the Nail R to the Ball and Socket S, whereby the Tube may be turned every way. The Centre of the flat Speculum CD must be placed in the same Point of the Tube’s Axis VT where falls the Perpendicular to this Axis drawn to the same, from the Centre of the Eye-Glass, which Point is here marked at T.

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 $$\frac{1}{6}$$ or $$\frac{1}{8}$$ Part of an Inch in depth, so that it magnifies about 40 Times in Diameter, which he thinks is more than any 6 Foot Tube can do with Distinctness; but (says he) by reason of bad Materials, and for want of a good Polish, it represents Things not so distinctly as a 6 Foot Tube will do; yet he thinks it will discover as much as any 3 or 4 Foot Tube, especially if those Objects be luminous; and says, he has seen with it Jupiter distinctly round, and his Satellites and Venus horned. He doubts not but in Time a 6 Foot Reflecting Telescope of this sort may be made, which will perform as much as any 60 or 100 Feet Refracting Telescope. That a Refracting Telescope, made of the purest Glass, exquisitely polished with the best Figure any Geometrician has or can design, would scarce perform as much more as an ordinary good Telescope of the same Length, &c. This is part of a Letter written by Sir Isaac Newton, from Trinity College, Cambridge, dated Feb. 23. 166$$\frac{8}{9}$$. to a Friend. See page 259. of Dr Desagulier’s Appendix to Dr David Gregory’s Elements of Opticks.