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Telefcope is faid to be 12 Feet, if the Semi diameter of the Object-glals, Convex on both Sides, be 12 Feet, i$c.
Since Myopes lee near Objeas beft ; for them the Eye- glafs is to be remov'd nearer the Objea-glafs, that the Rays refrafted thro' it may be the more diverging.
To take in the larger Field at one View, fome ufe two Eye-glaffes ; the formod whereof is a Segment of a larger Sphere than that behind : To this it molt be added, that if two Lens's be join'd immediately together, fo as one touch the other, the Focus is remov'd to double the Dillance which that of one of them would reach to.
To jhorten the Aflronomical Telefcope, i, e. to conftrnS a Telefcope jo, as that tho' Jhorter than the caramon one, it pall magnify as much.
l°. Having provided a drawing Tube, fit in an Object Lens EG (Fig. 43.) which is a Segment of a moderate Sphere; let the firit Eye-glafs BD be Concave on both Sides ; and fb placed in the Tube, as that the Focus of the Objea-glafs A may be behind it, but nearer than the Centre of the Concavity G. Then will the Image be thrown in Q_, fo that G A : G 1 : : AB : QI. LalUy, fit in another Objea- glafs, Convex on both Sides, and a Segment of a leffer Sphere, fo as that its Focus may be in Q.
This Telefcope will magnify the Diameter of the Object, more than if the Objea-glafs were to reprefent its Image at the fame Dillance EQ_; and conlequently a fliorter Telef- cope conlirufled this Way, is equivalent to a longer in the common Way. The Demonllration may be feen in Wolfus.
Sir Jf.Neivton furnilhes us with another Method of con- tracfing the Telefcope, in his Caloptrical or Reflecting Te- lefcope ; the Conttrucf ion whereof lee hereafter.
Land Telescope-, or T>ay Telescope ; a Telefcope con- fining of more than two Lens's, commonly of a Convex Objea-glafs, and thtee Convex Eye-glaffes : or, a Telefcope that exhibits Objeas erefl, yet different from that of Ga- lileo.
It has its Name from its being ufed to view Objecfs in the Day-time, or on the Earth, or from its being uied by Day.
To conflrucl a Land or Day Telescope.
A Tube being provided, lit in an Objea-glafs, which is cither Convex on both Sides, or Piano Convex, and a Seg- ment of a large Sphere : To this add three Eye-glaffes, all Convex on both Sides, and Segments of equal Spheres ; dif- pofing them in fuch Manner, as that the Dillance of any two may be the Aggregate of the Dillance of their Foci.
Theory of a Land Telescope.
Then will an Eye apply'd to the lad Lens, at the Di- llance of its Focus, fee ObjeSs very iiftinBly, ereS, and magnify 'd in the Ratio of the Tlijlance of the Focus of one Ere-glafs LK (Fig. 44.) to the Ttijfance of the Focus of the Objea-glafs AB.
For, i°. the Rays, from what has been already faid, fall- ing on the Object parallel ; the Image of the Object will be reprelented invcrtedly at the Dillance of the principal Fo- cus : Wherefore, iince this Image is in the Focus of the firll Eye-glafs, the Rays after a fecond Refraaion will be- come parallel, and thus falling on the third Lens, after a third Refraction they exhibit the inverted Image invertedly 5 that is, a direct Image of the Objea. Since then this Image is in the Focus of the third Eye-glafs, the Rays after a fourth Refraction will become parallel : And in this Dif- pofition the Eye will receive them, and confequently there will be diftinct Vifion, and the Objetl will appear eretJ.
2°. If IQ^=IK, that is, equal to the Dillance of the Focus of the Objea-glafs ; the Eye placed in M, will fee the Semi-diameter of the Obieft increas'd in the Ratio of LM to K I ; but the Ray A Q_, proceeding from the Focus Q_of the Object Lens A B, after Refraction becomes parallel to the Axis I L ; conlequently the firll Eye Lens CD joins it to the Axis in M, the Dillance of a Semi-diameter.
And fince the Focus of the fecond Eye-glafs EF, is alfo in M ; the Ray FH, after Refraaion, will be parallel to the Axis N O ; and therefore the third Eye-glafs will join it at the Axis in P; but the Semi-diameters of the Lens's GH and C D are iuppos'd equal ; therefore P O = L M. Where- fore fince the right Angles at O and L are equal, as alfo HO-=CL, the Angle OPH is equal to CM L. The Semi- diameter of the Objea, therefore, appears the fame in P as in M ; and is conlequently magnify \i in the Ratio of L M, crPOfoKI.
Hence, i°. An Adronomical Telefcope is eafily converted into a Land Telefcope, by uling three Eye-glaffes for one ; and the Land Telefcope, on the contrary, into an Aflrono- mical one, by taking away two Eye-glaffes 5 the Faculty of magnifying Hill remaining the fame.
2°. Since the Dillance of the Eye-glaffes is very final!, the Length of the Telefcope is much the fame as if you only ufed one.
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3'. From the Conllruaion, 'tis evident that the Length ot the 'Telefcope is had by adding five times the Semi-dia- meter of the Eye-glaffes to the Diameter of the Objea- glafs, it a Piano Convex; or its Semi-diameter, if Convex on both Sides.
Huygens firll obferv'd, both in the Aflronomical and Land Telejcope, that it contributes confiderably to the Perfeaion of the Inltrumenr, to have a Ring of Wood or Metal, with, an Aperture, a little lefs than the Breadth of the Eye-glafs, fix'd in the. Place where the Image is found to radiate upon the Lens next the Eye : By Means hereof the Colours, which ate apt to ditturb the Clearneis and Diilinclnels of the Objea, are prevented, and the whole Compafs taken in at a View, perfeaiy defin'd.
Some make Land Tekfcopei of three Lens's, which yet reprelent Objefls erea and magnify'd as much as the former. But fuch Telefcopes labour under very great Inconveniencies, both as the Objeas herein are tinged with falfe Colours, and as they are diilorted about the Margin.
Some again ufe four Lens's, and even more ; but fince fome Part of the Rays is intercepted in paffing every Lens, Objeas ate hereby exhibited dim and feeble.
RepiTrng, or Catoptric, or Cata-dioptric Telescope, is a Telejcope, which, inflead of Lens's, confifls chiefly of Mir- rors, and exhibits remote Objeas by Reflection, inltead of Refraaion. See CATorTRicics, g?&
This Indrument is the Invention of the great Sir If. New- ton: What determin'd him to apply his Thoughts this Way, was the different Refrangibility, which in his new Doarine of Light and Colours, he found the Rays of Light were of. In effea, as he found the Ratio between the greated and lead Refraaions of the different Rays, to be nearly as 28 to 27, it eafily follow'd, that the Rays could never be all refra&ed parallel from any Lens, but would fome of them divaricate more, fome lefs ; befide that, the Fnci would be dillurb'd ; the Focus of the moll refrangible Rays, being nearer the Lens than that of the lead refrangible ones, by a Dillance which is the 27th Part of the Didance between the Objea-glals, and the Focus of the lead refrangible ones. See Ray and Refrangihility.
Hence he concluded, that Refraaion was too unequal a Principle ; and that Lens's, of whatever Figures, whether Spherical, Parabolical, or any of the other , j.iic Seflions, and how truly foever ground, would never fuffice for the Perfeaion of Telefcopes.
Upon this he had Recourfe to another more equable Principle, viz. RefleBion ; and made a Telefcope, confiding of Specula, or Mirrors : The firll Hint whereof he owns he took from Dr. Gregory's Opticks.
ConjtrvMion of a RefteBing Telescope.
Provide a Tube A B C D, (Fig. 4$.) open in A D, and clos'd in BC, well black'd within-lide, and of a Length equal to the Didance of the Focus from the Concave Spe- culum EF. To the Bottom BC, is to be fitted a Concave metallic Speculum ab, polifh'd to the greated pofiible Per- feaion ; or rather, to have the Objeas clearer, and more diftina, let it be a Glafs Speculum, Concave on its fore Side, and equally Convex on the hind Side ; lor unlefs it be of the fame Thicknefs every where, it will reflea the Images of Objeas tinged with a fpurious Colour, and in- diflina. Towards the other End of the Tube is fix'd atl Iron Piece HL, to which is cemented a plain metallic Spe- culum ; or, which is better, a triangular Prilm of Glafs or Cryllal G, whofe upper Angle G is a right Angle, the two others half right. The Faces or Planes that meet in the Angle G, to be fquare, and the third a Parallelogram. This Prilm is to be fo difpos'd, as that a Ray refleaed from the Speculum, paffing thro' the Middle of the Face GM, may cut it at right Angles ; but be inclin'd to the Reaangle MN in an Angle of 45°. Its Didance from the Concave Specu- lum EF, is to be fuch, as that the Rays ac and hd refleaed from the Concave Speculum, may, after a fecond Refleaion from the Bale of the Prifin, concur in the Point e ; that is, the Didance of the Focus e from the refleaing Surface of the Prifm, and the Didance of that from the Concave Spe- culum, is to be equal to the Dillance of the Focus from the Concave Speculum. In I is placed a Piano Convex Lens, whofe Focus is in e, that the refleaed Rays may enter tho Eye parallel. Laflly, this Lens is cover'd with a thin Brafs or Leaden Plate, having a little round Perforation therein, for the Eye to look thro', by which means all foreign Rays ara excluded, which would otherwife occafion Confufion.
In the firft Telefcope of this Kind which the Inventor made, the Semi-diameter of the Concave Metallic Speculum was 1 2 f Digits, or Tenths of an Inch ; from which, therefore, the Focus was 6 i Digits didant. TheDiameterof the Eye- glafs was I of a Digit ; fo that it magnify'd the Diameter of the Qbjea in the Ratio of 1 to 38 : But he found that Objeas were fhewn fomewhat obfeure hereby ; on which Account, he afterwards recommended Glafs Specula inflead E B b b J f of
