Mathematical Instruments
Suppl. Ch. VII.

Of Sir Isaac Newton’s Reflecting Telescope, as improved by Mr. Hadley, and presented by him to the Royal Society.

Fig. 18

The Instrument (Fig. 18.) consists of a metaline Speculum, about six Inches in Diameter; the Radius of the Sphere on which it’s concave Surface was ground is 10 Feet 5\(\frac{1}{4}\) Inches, and it’s focal Length is 62\(\frac{5}{8}\) Inches. The back has a hollow Screw made at it’s Centre to receive the End of a Handle which is screwed on wherever the Metal is to be moved, in order to avoid bullying it’s polished Surface by handling. The Object-Metal A is placed in one End of an octangular Tube BB about 6 Feet long, dyed black on the Inside; about 6 or 7 Inches in Length of the three uppermost Sides of the Tube C towards that End at which the Metal is placed, are seperated from the rest, and open with two Hinges to make room for the Metal to be put in, or taken out; the End of the Tube is closed by an octangular Piece of Board D, which has an opening about \(\frac{2}{3}\) of an Inch broad, from the Top, down to a little below the Center, to give room for the beforementioned Handle, when the Object-Metal is lifted into, or out of, the Tube; at other Times it is closed with a sliding Shutter; the Metal is placed so as to have it’s Axis coincide with that of the Tube, by the Means of three small Buttons fixed to the Inside of the Tube, having their hinder Ends all in the same Plane to which this Axis is Perpendicular; two of these appear at aa, the third being at the middle of the Bottom of the Tube, is not seen; the foreside of the Metal rests against these Buttons in three Points of it’s Circumference, nearly equidistant from each other, and is held in them by three Screws, one of which appears at b, which run through the octangular Board at the End of the Tube, and bear against the Back of the Metal, in three Points, which directly answer those three on the foreside, with just so much Force as is requisite to keep it steady in it’s place; they must not be screwed harder against the Metal for fear of bending it, which, though it is half an Inch in Thickness, a very little Force is sufficient to do: when the Instrument is not used, these Screws are loosened, and the Object-Metal is taken out and laid by to prevent it’s tarnishing.

Fig. 19

The oval Plane (Fig. 19.) is composed of a Plate of the same Metal with the great Speculum, about \(\frac{1}{15}\) or \(\frac{1}{10}\) of an Inch in Thickness, soldered on the Back to another of Brass; it's Breadth is something less than half an Inch, and is in the Ratio to it’s Length, As 1 is to the square Root of 2, or as the Side of a Square is to it’s Diagonal 5 at one End of the Oval, the Brass Plate projects a little beyond the other, and has a Screw cut through it in that part, as also another directly against the Centre of the foreside; the other End is cyphered away on the backside, that it may intercept as few of the Rays in their Passage towards the Object-Metal as possible. The two Screw-Holes in the back (Figure) serve to fix this Oval A, to a Brass Arm B, which is fastened at the other End into a Slider EE (Fig. 18, 19.); this Slider is of an equal Thickness with the Side of the Tube, and has a Groove GG (Fig. 18.) cut for it in that side parallel to the Axis, and long enough to give room for it’s Motion, to set the Speculums at the different Distances, which the several Eye-Glasses require; it rests on the Inside against two thin Ledges, fastened within the Tube along the Sides of the Groove; on the Outside, it is kept in it’s place, by a sliding Shutter not expanded in the Figure. In the middle (Fig. 19.) it has a cylindrical Cavity D, whose Axis is perpendicular to it’s inner and outer Surfaces; each of the Boxes in which the Eye-Glasses are contained, is fitted to this Cavity; the beforementioned Brass Arm is fixed into the Inside of this Slider, towards the End farthest from the Object-Metal; it rises perpendicularly for about two Inches, and is made flat, so as to turn one Edge to the Rays which come from the Object; about b it is bent forwards, and flatted the other way, so that when the back of the oval Plane is held flat to it by the two Screws cc, the Axis of the cylindrick Cavity may fall on the Centre of it’s foreside, inclined to it’s Surface in an Angle something less than 45 Degrees; this Angle is brought to be exact by two very small Screws ii, whose Threads take hold in the flatted End of the Brass Arm, and their Points bearing against the back of the Oval, raise one End of it a little from the flat of the Arm. The Speculums are set at their due Distance by turning a long Screw CC, for which there is a Nut lodged in the Slider at g; the Screw is kept from moving backwards or forwards, when it is turned, by a Brass Plate F, which is to be fixed to the flat End of the Side of the Tube, and taken off at Pleasure. Each of the Eye-Glass Boxes H, has a Screw on the outward End to fasten to it a Bowl or Dish I, to receive the Ball of the Eye, and guard it from external Light.

On the Top of the Tube a common Dioptrick Telescope H is fixed upon two small Pillars, about 18 Inches long, with it’s Axis parallel to that of the Tube; with two small Hairs in the common Focus of the Object and Eye Glasses; eroding one another in it’s Axis.

There are three convex Eye-Glasses belonging to the Instrument; the first, or shallowest, has it’s focal Distance of about \(\frac{1}{3}\) of an Inch; the second of \(\frac{3}{10}\); and the deepest of \(\frac{11}{40}\); or something less. When the first of these is used with the Instrument, it magnifies about 188 s or 190 Times in Diameter; with the second, about 208, and with the third about 228, or 230: Each of these Glasses has placed in that Focus nearest the Oval, a Circle to determine the part of the Object seen at one View; and in the other Focus towards the Eye, a Brass Plate with a little Hole in the middle, to let no Light pass to the Eye from the Inside of the Tube, but what comes from the Oval: besides these three convex, there are two concave Eye-Glasses with which it magnifies about 200, or 220 times; and also a Set of three convexes, which turn it into a Day Telescope, magnifying about 125 times. The Aperture is limitted by a Circle of Card or Paste-board placed before the Object-Metal in the Tube. To vary the Apertures, there are three of these Circles, and the Apertures altered by them are five Inches and a half, five Inches, and four and an half, though for some Objects, the whole Tube may be left open.

Thus far is the particular Description of the Body and main Parts of the Telescope, as is to be found in the Philosophical Transactions, Numb. 376. As to the particular Description of the wooden Support to direct the Tube to any Object which is there given I shall omit, the Figure thereof being sufficient to satisfy all those who have not such a Telescope, and have no Desire of making it; besides, it is in any one’s Power almost to invent a Support for the Body of the Instrument, and that, perhaps, different from this here, of Mr Hadley’s; there is no great ingenuity in doing it.

Mr Hadley tells us, this Instrument when tried at an Object seen in the Day-Time, seems to bear an Aperture of five Inches and an half, with the deepest of the aforementioned Eye-Glasses, as well as the common Telescopes do the usual Charge and Aperture given to them, except that in these the Objects appear a little brighter.

Drs Pound and Bradley say, that this Telescope will bear such a Charge as to make it magnify the Object as many Times as Mr Huygens’s dioptrick Telescope, the focal Length of whose Object-Glass is 123 Feet, and that it represents Objects as distinct, though not altogether so clear and bright.

1 have always thought, that these catadioptrick Telescopes of both Kinds, do represent Objects more obscurely than refracting Telescopes, though indeed in this the Newtonian one has the Advantage of Dr Gregory’s; and that they will always do so, unless a reflecting Substance for a concave can be found, that will reflect as much Light as Glass transmits, which I take to be impossible. For I ever saw the Image of an Object in the Focus of a concave Speculum more indistinct and obscure than the Object itself, (which argues the Loss of Rays by the Reflection) or than an Object appearing by the Reflection of a plane Speculum; whereas an Object seen through a Glass-Lens will appear more enlightened and bright than even it does with the naked Eye.

Mr Huygens says, that he had never found any Speculums that had so good a Polish as Glass; and so says Sir Isaac Newton, and accordingly he recommends glass Speculums quicksilvered over, as preferable to metalline ones.

In the second Volume of Dr Smith’s Opticks, beginning at pag. 363. there is a Reflecting Telescope of Sir Isaac Newton’s Form, made, and described by Samuel Molyneux, Esq; and sent by him as a Present to his Majesty John V. King of Portugal, being of a much Ampler Construction than that of Mr Hadley’s, and magnifies as much, and as distinctly, as a Refracting Telescope of 35 or 40 Feet. So says Dr Smith.

And at Numb. 918, pag. 36;. there follows a very strange and Angular Assertion of Dr Smith’s, viz. he says, that one’s Imagination is always misled in comparing the Effects of this Instrument with Refracting ones of that Length. For although (says he) by the Minuteness of the Parts viable in the Object observed; as also, by the Proportions between the focal Distance of the Eye-Glass made use of, and the focal Distance of the great Speculum it is demonstrable, that the Distinctness and magnifying Power thereof exceeds that of a Refracting Telescope of 35 Feet, yet the Spectator will always fancy that the Refractor has the Advantage of this Instrument. And again, that it will be certainly found to be but a Deception, and probably an universal one to all Persons. Now, is not the Doctor here rather deceived himself. For to prefer optical Demonstrations, founded upon dubious Principles (which I think his are, about the apparent Distinction, and brightness of Objects) rather than to the Experience of all Men, is too bold an Assertion. And one would be almost led to believe, that the Doctor would have Reflecting Telescopes, right or wrong, to be better than Refracting ones, as to the Distinctness of the Objects appearing through them.

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