702

TEL

T E L

use of glasses known to the world; but our countryman, FriarBacon, who died twenty-one years before him, was, in all proba-bility, acquainted with them first; for he wrote a book of per-spective, in which he plainly shews that he did not only under-stand the natme of convex and concave glasses, but the use ofthem when combined in telescopes; though he nowhere, in thattreatise, discovers the manner in which they are to be put toge-ther. The telescope, with the concave eye-glass, was first in-vented by a mechanic of Middleburgh in Zealand, called Z.Johnnnides, about the year 1590, though .1. Lipperhoy, anotherDutchman, is candidate for the same discovery. Gideon Harvey,in a book printed in 16G2, says, that Jacob Melius, of Alcmaer,about forty or fifty years before first invented the telescope by ac-cident. I'ranciscus Fontana, a Neapolitan, contends, that he wasthe first contriver of the telescope composed of two convex glasses,which is now the common astronomical telescope ; and Kheitapretends to be the first that rendered that telescope fit for terres-trial uses, by adding two eye-glasses to it. The telescope is oftwo sorts, viz. dioptric, or refracting ; or cala-dioptric, by reflec-tion and refraction conjointly.

Telescope, Dioptric or Refracting , consists of an object-glass a'c, (Plate CLXXII, fig. 1.) by which the image fd of anobject OB, at a distance, is formed in the focus e of the said glass,and in an inverted position. This image may be viewed by asingle lens, ab, placed at its local distance, as is usually done forviewing the heavenly bodies, because in them we do not regardthe position ; but for viewing objects near us, whose image wewould have erect, we must, for that purpose, add a secondlens pq, at double its focal distance from the other, that the rayswhich come from ab may cross each other in the focus o, in orderto erect the image In, which it will form in its own focus m, be-cause the rays come parallel from the tiiNt lens ab. Lastly, athiul lens ic is added, to view the secondary image In. Tiiesethree lenses, or eye-glasses, are usually of the same size and focallength ; and the power of magnifying is always as the focal lengthof the object-glass ca divided by the focal length of the eye-glassIn or he: for instance, suppose etc = 10 feet or 120 inches, andhe or Im = 3 inches; then will the object appear to the eye,through such a telescope, 40 times bigger than to the naked eye;and its surface will be magnified 1G00 times, and its bulk or soli-dity 64000 times. If instead of a convex eye-glass we should use aconcave one of the same focal length, it would represent the ob-ject erect, equally magnified, and more distinct and bright; butthe disadvantage of this glass is, that it admits but of a small area,or field of view, and, therefore, not to be used when we would seemuch of an object, or take in a great scope; but it is used togreat advantage in viewing the planets and their satellites, Saturn’sring, Jupiter ’s belts, Sec. The magnifying power of a refractingtelescope is thus estimated: let AB, (fig. 2,) be the object-glass,and CD the eye-glass; and let HFI and GFM be two rays comingfrom the extreme parts of a distant object, and crossing each otherin the centre F of the glass AB. Then is the angle G1TI = [ FMthat under which the object appears to the naked eye ; but 1F.M= CKD is that under which the image appears as magnified bythe eye glass CD. But the angle IEM is to the angle IFM, asLF to LE, or as the focal distance of the object-glass to the focaldistance of the eye-glass; and in that proportion is the object mag-nified.

Telescope, Cata-dioptric, or Reflecting, is representedin fig. 3; ABEII is the large tube, cr body of the instrument, inwhich BE is a large reflecting mirror, with a hole in the middle-CD. This mirror receives the rays ac, bd, coming from the ob-ject at a distance, npd reflects them converging to its focus e,where they cross each other, and form the inveited image IM ; xyis a small concave mirror, whose focus is at /, at a small distancefrom the image. By this means the rays coming from the imageare reflected hack through the central hole CD of the large mirror,where they fall oil the plano-convex lens WX, and are by it con-verged to a focus, and there form a second image RS, very largeand erect, which is viewed by a meniscus eye-glass YZ, by theeye at P, through a very small bole in (lie end of the eye-pieceA CDZ. If the first lens WX were taken away, the image wouldbe formed somewhat larger at MI; but the area or scope wouldhe less, and therefore the view not so pleasant. At TV is placeda circular piece of brass, with a bole of a proper size to circum-

scribe the image, and cut off all superfluous or extraneous rays,that so the object may appear as distinct as possible. This formbeing contrived by Dr. Gregory is called the Gregorian telescope.As the image is formed by reflection, the rays of every sort willbe united nearly in one point, amt will therefore admit of an eye-glass YZ of,a deep charge, or small focal distance; and so thepower of magnifying will be proportionally increased. The mag-nifying power of a .reflecting telescope is thus computed. Theparallel rays KB, (fig. 4,) and LE are reflected by the large ob-ject-speculum AF to Its focus a, where the image IM is formed;which image is defined by two other rays NQ, PQ, coming fromthe extreme parts of the object at a remote distance, and meetingin the centre of the large speculum at Q. Now if / be the focusof the small mirror Gil, supposing the image were formed in thesaid focus/ (that is, that both the foci a and / were coincident)then the lays proceeding from the image IM will proceed parallelafter reflection, and produce distinct vision of the image, whichwill then subtend an angle IOM at the centre O of the speculumGH, which is to the angle IQM, under which the object appearsto the naked eye, as «Q to «0 or JO. So that the magnifying

power would in this case be as

aQ_

JO

But to increase this magni-

fying power, the image IM is not placed in the focus of the smallspeculum, but at a small distance beyond it, as is represented inthe figure ; by which means the rays coming from the image to(be speculum GII will be reflected converging to a distant focus R,where a secondary large image IM is formed from the fir;t imageIM ; which image INI is seen under the same angle IOM withthe former from the centre of the speculum GII, but from thecentre of the eye-glass TV it is seen under the large angle ISM.But the angle ISM is to the angle IOM as OR to SR; wherefore

the second ratio, or part of the magnifying power, is that of -^2.

Consequently, the whole magnifying power of the telescopeis -^y x (because in this case/O becomes «0.) Or, in

other words, the angle NQP, under which the object appears tothe naked eye, is to the angle ISM, under which the large magni-fied secondary image IM appeal's to the eye through the eye-glass,

aQxORas «o x SIT

A small alteration was made in the structure of this

tele-copeby Mr. Cassegrain, viz. in using a convex speculum GII,(fig. 4,) instead of the concave one GIJ. Now if they are equallyspherical, that is, if they are segments of the same sphere, thenwill/be also the virtual focus of the convex GH ; and, if all othertilings remain the same, the first image IM will be virtually thesame as before, and the last image IM will be really the same; sothat the magnifying power of this form of the telescope is

r 7(Tx/n V w *" c *’ '* et l ua * to D'iR °f Gregory’s form. Sir Isaac

New ton had this telescope made in a different form or manner, asfollows: ABCD, (fig. 5,) was a large octagonal tube or case ; EFa large polished speculum, whose focus is at o ; GI1 a plane specu-lum truly concentered, and fixed at half a right angle with the axisof the large one. Then parallel rays a E, b F, incident on the largespeculum EF, instead of being reflected to the focus o, were inter-cepted by the small plane speculum GII, and by it reflected to-wards a hole cd in the side of the tube, crossing each other in thepoint O, which is now the true focal point: and from thence theyproceed to an eye-glass rf placed in that hole, whose focal dis-tance is very small, and therefore the power of magnifying may bevery great in this form of the telescope ; because the image IM ismade by one reflection (for that of the plane speculum only altersthe course of the rays, and adds nothing to the confusion of theimage) and will, for that reason, hear being viewed by a glass of avery deep charge, in comparison of an image formed by differentlyrefrangible rays.

Telescofe, Solar. This instrument is applied to use in thefollowing manner: AB, (fig. 6,) represents a part of the window-shutter of a darkened room, CD the frame, which (by means of a

screw;