Arms. By this Time I fee the British Ifle is juft upon entering the illumined Hemifphere, and the Index now points to about 3 after II : But the Cape of Good Hope is ftill in the Dark.And juft now I perceive the artificial Planet, or Patch, just rifing above the Horizon, and so far advanced on the Solar Difk as it appeared to those who had the Pleafure of viewing that Part of the Phænomenon. As the Motion of the Earth continues, the Parts of the Surface are fucceffively brought into, and carried out of the enlightened Hemifphere, and England, amongst the reft, gradually proceeds towards the middle Part of its diurnal Arch:-In the mean Time, the Planet regularly advances on the Sun's Difk,————— and is nearly in the Middle of her Path when the Index of the Globe points at VI.The Cape now enters the Sun-beams, and obferves the Tranfit more than half over, while the Sun is not yet rifen to St. Helena. The Earth keeps revolving uniformly upon its Axis, and, at length, brings St. Helena to enter the enlightened Hemifphere, and foon after views the End of the Tranfit at a fmall Altitude of the Sun above the Horizon.———I obferve the artificial Planet to be now much about that Pofition of the Sun's Difk as when I firft faw it through the Telescope, viz. about feven o'Clock in the Morning of that memorable Day. - The Motion here of the

Planet, I plainly fee, is every Way fimilar to that of the real Planet on the Sun's Disk. The Motion of the Planet and the Earth being perfectly correfpondent to each other, at length the Patch arrives at the western Limb of the Sun, and fhews the internal Contact of the Planet at the Time the Index points to a few Minutes after VIII in the Morning.And it is very curious now to obferve the artificial Egrefs, which feems in every Refpect to bear a proper Refemblance to that of the natural one.

-The artificial Tranfit is just now at an End, for the Patch is now in external Contact with the Limb of the Sun. The Index of the Globe now points to nearly half an Hour after IX, fo that I obferve the Time of the Egrefs here is about eighteen Minutes, which is the fame as was obferved of the real one- In the laft Place, I fee the Pofition which our Ifland had on the enlightened Hemisphere at the Time of the total Egrefs, and alfo the rela

tive Situation of all other Places where that Phænomenon could be feen.And now, Cleonicus, I muft acknowledge that you have, by this Means or Contrivance, conveyed to my Mind as perfect a Knowledge of the geographical and optical Principles of the Tranfit, as I could poffibly have from any artificial Machinery, and, for your Care and Trouble herein, fhall ever think myself highly

indebted.

Cleon. It ever was, and will be my Study to reprefent the various Phænomena of Nature in fuch a Manner as may moft exactly correfpond with the Operations of Nature itself; and nothing will more felicitate my Succefs than the Pleasure and Inftruction that may thence redound to one who is fo dear to me as yourself. I fear I have, by this Time, proved tedious; but you will remember, it is upon a Subject that, like the fecular Games, happens but once in an Age.

DIALOGUE XII.

Concerning the NATURE and USE of BINOCULAR TELESCOPES.

Cleonicus.

UR laft Converfation was a Digreffion from our Speculations on the Nature and Ufe of optical Inftruments. But it happened very opportunely to a moft fignal Inftance of the latter, as I then obferved to you.

I fhall now return to the former Subject, which I shall re-aflume with the Confideration of the Nature and Ufe of Binocular Telescopes. We have already largely enough defcanted on the common Conftructions of this Inftrument; but as the Binocular Telescope is of a different Form, and has a very peculiar Property, I thought it might be worth while to fpeculate on the Nature of it for an Hour.A French Author, formerly, (P. Cherubin) had fo high an Opinion of this Inftrument, that he wrote a large Volume in Folio almost wholly upon the Subject;

and,

and, indeed, ever since their first Invention, their Effects have been greatly admired by all who delight in Optics.

Euphrof. If I can judge from the Sound of the Word, this Inftrument employs both the Eyes; and as I prefume the Inftruments you fpeak of, are thofe before us, they appear to me to be a double Telescope, one of the refracting, and the other of the reflecting Kind.

Cleon. You obferve very rightly in each Refpect; the Binocular Telescope confifts only of the Glaffes of two common Telescopes, properly adjusted, to fhew the fame Object to both Eyes. You will readily conceive the Defign and Utility of this Conftruction, by confidering how great the Advantage of common Vifion by two Eyes, is, in Comparifon of that by one only: It hath been usually made an Argument of a moft obvious Providence, that Animals are in general furnished with two Eyes, that they might not be dark with the Lofs of one; but we may add to this, the great Pleasure and Perfection of viewing Objects with two Organs of Sight instead of one: Nor is this at all difficult to be experimented, as I make no Doubt you have oftentimes had the Curiofity

to try.

Euphrof. I have many a Time tried that Experiment, and found the Difference very fenfibly :-The Field of View is not only more confined by fhutting one Eye, but the Objects are lefs vivid and bright, and feem, as it were, diminished and contracted to the View.

Cleon. It is all very true that you fay; but it is very obfervable, that the Difference is nothing like fo great to the naked Eye as by the Telescope, where, as we may fay, this very Difference is magnified in Proportion with the Objects themselves; and this is that very wonderful Effect that I juft now mentioned, and which fo ftrongly recommends this Inftrument to all the Curious.

Euphrof. But how are the two Telescopes adapted to exhibit one Idea of the Object only: One would think, that, by two Telescopes, we should have two different Views of the fame Object?

Cleon. The natural Reafon of this is the fame as that by, which an Object appears but one and the fame to two naked Eyes, notwithstanding there are two Images formed, one in the Bottom of each Eye. For by the wonderful

Difpofition

Difpofition of the nervous System, the two optic Nerves are united in the fame Part of the Brain, where the Senfations of each Image, or the Ideas which they occafion, are united in one, and fo prefent it to the common Senfory of the Mind; and you will eafi'y conceive that the Telescope, fingle or double, does not multiply the Image, but magnifies it only. The Image in each Eye is ftill the fame, but only larger; and therefore two Telescopes only produce two larger Images to the Eyes, inftead of two fmall ones, and confequently the Mind has still the fingle Idea of the Object, but enlarged.

Euphrof. I prefume, this is pretty much a parallel Cafe with those who ufe Spectacles; for they, with two Glaffes, fee the Object enlarged, but fingle at the fame Time.

Cleon. Your Comparison will hold, provided the Spectacles you fpeak of are difpofed as they ought to be, (but thofe of the common Make are not,) viz. when the Axis of the Glaffes are both directed to the Object that we view, and there interfect each other: And this is the Cafe of the two Telescopes which compofe this Inftrument, the Axis of each are fo directed, by adjusting Screws, as to point to, and terminate in the Object we look at; for, in that Cafe, each Telefcope prefents the fame View, enlarged, as we have in fmall, by the naked Eye: Thus, two Telefcopes, in that Respect, have the Effect of one, with this important Addition, that the Field of View in the Binocular Telefcope, though really the fame as in the fingle one, is yet feemingly vaftly enlarged, at a greater Distance, fhewing the Objects more ftrong, bright, and perfect, than they ufually appear; which is one of the most agreeable and noble Deceptions that can be found in any optical Inftrument, of which I fhall now gratify you with an Experiment, having first obferved to you, that while we look through the Inftrument it is neceffary to turn two Sets of Screws, to adjust it to the Eye, one which moves the Object-glaffes, and the other the Eye-glaffes, nearer to, or farther from each other, 'till the Axes of each Telefcope coincide with thofe of the Eye and nicely meet in the Object.In this Manner I have now adjufted the Refracting Binocular, and now, if you take my Place, you will

fee

fee yonder House in the most agreeable View you ever beheld it.

Euphrof. I do, fure enough.- I find every Thing just as you have defcribed it: -With both my Eves I fee the Field of View, in Appearance, wonderfully enlarged: The Appearance of the Houfe more natural, eafy, and vivid. -But when I fhut one Eye, then I fee the very great Difference you speak of, the Field, as it were, feems contracted as well as the Objects; and every Thing has a poor and unpleafant Afpect in Comparifon of the former.- To tell you the Truth, Cleonicus, you will, by this Experiment, put me out of Conceit with the common Telefcopes. fcarcely think I should reafon well if I used a fingle Telefcope inftead of a double one, any more than I should, if I preferred viewing Objects with one Eye inftead of two.-Pray, is the Effect nearly the fame in the Binocular Reflectors?

-I can

Cleon. It is there in Proportion heightened, as they magnify more than the Refractors I have fitted them for your View :- -Look now at the fame Object through them:

may

Euphrof. Well, this is wonderful indeed :I truly fay I never knew what it was to view Objects thro' a Telescope before. I have feen them indeed magnified, and thought that was a wonderous Effect; but I fcarcely know how to exprefs the different Senfations which arife from a Profpect of the fame Objects in this compounded View:-Here is truly (je ne fçay quoi) fomething inexpreffibly delicate, grand, and delightful in the Effect of this Inftrument!It is amazing to me, that every Lady or Gentleman of Fortune and Curiofity has not one of thefe Binoculars in their Poffeffion.

Clean. You might be in an eternal Amazement if fuch Things were to excite it; the Cafe is fo far otherwise, that you feldom obferve the Purchase of any Inftruments is in Proportion to their Merit and Ufes: Things in common Vogue, that give us no Trouble to understand their Ufe, and which have been made Time immemorial, are moftly the Subject of common Demand; few inquire after new, or more perfect Improvements.But yet, I cannot help wondering a little, with you, that there are VOL. II.

T

not