DIALOGUE VIII.

The NATURE and CONSTRUCTION of refracting TELESCOPES continued.

I

Euphrofyne.

Am now ready to attend you, Cleonicus, in what you have further to say upon the Subject of refracting Telefcopes. You have already defcribed to me the four principal Forms, the laft of which was one of four Glaffes; but I remember you faid, there were other Telescopes that had more than four Glaffes; please to let me know what Purpose the additional Glaffes answer?

Cleon. In fome there are five, and in others there are fix Glaffes, which are principally intended to enlarge the Field of View, and to render the Image more perfect towards the extreme Parts: The Difpofition of the Glaffes is different in many of them; but, in general, it is the fame as what you fee in this, which I here put into your Hand:And that you may fee the Difference between this, and a four-glafs Telescope of the fame Length, you have now an Opportunity of viewing the Crofs, the Globe, or Ball, and the Dome of St. Paul's, the Sun lying advantageously upon them.

Euphrof. Thefe Telescopes, I fee, are about three Feet in Length, and I fhall gladly experience their Difference. Firft, I will take the four-glafs Telescope, with this I fee the golden Crofs, curioufly wrought, and very large and distinct.——— -The Ball appears very grand, as well as globular, with curious Workmanship, and the Golden-gallery appears like what it is when we are upon it; I can eafily diftinguish Men and Women, their different Habits, Colour of their Cloths, fome of them walking, and others leaning over the Palafades.The Dome appears to be glorious Rotundo, and every other Part in Proportion, exhibits an agreeable Spectacle,

-I will now take the Telescope with fix Glaffes, -and, in viewing the fame Objects, I perceive a Difference in the Extent of the View, which is certainly here much larger and finer than before; but,

very

at the fame Time, I do not think the feveral Parts appear fo very diftinct as in the former,and, on that Account, I really do not know which of the two I should chufe.

Cleon. You need give yourself no Trouble about that, my Euphrofyne; for I fhall make you a Present of them both :- -They have each their peculiar Advantages, and you never will find any one Inftrument in which the different Perfections of various Forms can be all united.

Euphrof. I readily believe what you fay: But what are thofe long Telescopes in Mahogany, and coloured Wood?- Are they not what you call Sea-Telescopes, Cleonicus?

Cleon. Yes, my Euphrofyne, they are, and used chiefly by Mafters of Veffels, and other Sea-faring People, for as they are all in one entire Length, they give them no Trouble, and require no Time in applying them to ufe: This renders them more fit for their Purpose, who oftentimes have not a Moment to fpare, when their Ufe is required, for viewing a Ship, Headland, Lights, &c. at the first Appearance. Add to this, that Vellum, or Parchment Tubes, of which common Telescopes are made, will not bear the Action of a faline Atmosphere, which foon difcolours them, deftroys their Texture, and renders them unfit for Ufe, unlets they are taken very great Çare of indeed, and kept in proper Cafes.

Euphrof. But I fee here a Telescope fomewhat different from the former, especially at the Object-end, the Brafswork of which I perceive to be of a fquare Form; to what Puipofe does that ferve, Cleonicus?

- Cleon. That Object-end of the Sea-Telescope is made in the Form of a Dye, on Account of its holding two Glaffes; either of which may be immediately applied, as an Object-glafs, by turning the Place round in which they are contained, which is done in an Inftant, by a proper Mechanifm in that Part; by this Means you have, as it were, a double Telescope in one Inftrument: for the Glaffes being of a different focal Diftance, and the Brafs Tube being made to draw out at the Object-end as well as the Eye-end, the Telescope may be lengthened, or hortened, at Pleasure according to the Object-glafs that

is used:-But when we confider, that fuch a Telescope becomes, by this Conftruction, fo expenfive, as to equal the Price of three very good fingle Telescopes, at the fame Time each of which may have a different Power of magnifying, it appears to me to be a very prepofterous and an injudicious Contrivance, and, as fuch, deferves no further Notice.

Euphrof. There is yet another Telescope which makes a confiderable Appearance, concerning which you have not faid any Thing. Pray, is the internal Structure of this different from any of the former, as I fee nothing different in its external Form?

Cleon. This is a Patent Telescope, and different from others in having a double Object-glafs, one Convex and the other Concave, placed contiguous to each other in the fame Cell: The spherical Figure of which is different in each, and the Defign of fuch a Combination of Glaffes is, that one may correct the Errors of the other, in regard to the colorific Refraction of Light; for whereas you have been taught by prifmatic Experiments, that Rays of Light have different Degrees of Refrangibility, and I have not long fince obferved to you, that this is one great Cause of the Imperfection of refracting Telefcopes with a single Object-lens of a Convex Form, therefore this Concave is added to correct this Aberration of Rays, and to render the Image of external Objects more perfect and uniform.

Euphrof. And do thofe two Glaffes anfwer fuch a Purpose in a fenfible Degree?

Cleon. You will have no Occafion to take my Word, or Opinion, about this Matter, because you may try the Experiment yourself, and fee the Difference betwixt this and another of the fame Length and magnifying Power. I have here contrived to place them both in a Pofition, to view the fame diftant Object, which happens very apropos to be the Crofs on the Top of St. Paul's in a direct View.

Euphrof. The Crofs makes a very curious Spectacle ; and I obferve, very plainly, that there is a Difference, but not fo great as I fhould have imagined, from the very great Difference which you mention in the Price of thefe two Telescopes.- There is (je ne fcai quoi)

fume

fomething which I cannot well exprefs, in the Effect of this Telescope.I fee the Crofs and its Parts more perfectly in one respect, yet not so much fo in another.

But there feems to be fo much of Ambiguity in the different Effects of thefe Telescopes, that I must be longer exercised in the Comparison of them before I can pretend to form any adequate Notion of their respective Merits, or how much one is preferable to the other. But, pray, what is that particular Construction of a Telescope you call a Polemofcope?

Cleon. By that Term is meant, a Telescope, at the Object-end of which is fitted a very curious reflecting plain Speculum at half a Right-angle to the Axis of the Telescope, which is directed to the middle Point of the faid Speculum, as you observe in this Part which I screw off and on.

Euphrof. But what is implied in the particular Etymology of the Word?

Cleon. The Name implies an Inftrument of the Telefcopic-kind, for the Ufes of War; that is to fay, fuch as the Generals, or Officers concerned in Sieges, Fortifications, & have often Occafion to ufe: For, in many Cafes, befides a direct View ftrait forward, they have often Occafion to fee what is doing by Perfons on either Side, above, or below the Place where they ftand, and which, with this Contrivance, they can eafily command a View of; for, as you turn the Glass in your Hand, you take in all Objects in a lateral Situation, or fuch as are placed Side-ways, and cannot be feen by a common Telefcope.If you turn the Speculum downward, you fee all below ;- if you turn it upwards, the Top of the Buildings, and the People there, are expofed to your View: And thus the Actions of the Enemy may be reconnoitred, while they think themselves quite fecure from your Notice.

Euphrof. Why, after all your learned Terms, I find that this high-founding Inftrument is nothing more than the common Opera-glafs, which a Lady of my Acquaintance fhewed me the other Day, having a Hole on each Side, in which there appears a plain Looking-glafs adapted to each End, and by which fhe could take a View of any Perfon the pleased without his having the least Suspicion

of