TELESCOPE. 



Uie other eye, bciug alfo ufed, but cat of the tclefcope, 

 projeds lliat imagu- upon a liorizontal line, bounded by foine 

 oblervablo diftiiiCt nniks, that can be knov !i again : then 

 as often as the angle lubtended at the place uf obfervation 

 by tlic objca, of which the image was obferved, is contained 

 ill the angle fubtendcd by tlic horizontal line into which 

 it was projeded, fo much docs the image apparently exceed 

 the objcd ; i. c. fo much does the tcleicope magnify. For 

 inftance, fup|)ofc that forty bricks in the wall of a diftant 

 building appealed juil to occupy the whole field of view of 

 any tclefcope, and that the angle fubtended by thofe bricks 

 meafured 50', by aiiy other inftrument, which would be 

 the cafe at the eighth of a mile very nearly ; then fuppofe 

 that the horizontal line covered by the forty magniiied bricks, 

 or by the luminous circle of the field of view, was bounded 

 by two trees, and fubtended an angle of 41° 40' ; on Uiefe 



41° 40' 

 fuppofitions, the power would be ^ — = 50 : and in 



this way, but not fo well as by the pearl dynameter, a tele- 

 fcope compofed of any number of glaflcs, or fpecula and 

 glafles, may have its power determined, without any regard 

 to the radii of the glafTes or fpecula, or to their refpeftive 

 pofitions. Should the refults of the two mctliods accord, 

 it may be taken for granted, that the determination arifing 

 from the average is fufficiently accurate. The powers of 

 the four telefcopes with the fame eye-piece, which we have 

 beTore mentioned, were taken again by this fecond method, 

 and found to be as in the fubjoined ftatement ; viz. 



In. 



30.15 focus 

 45.75 ditto 

 63.5 ditto 

 1 18.8 ditto 



= 30-9 



47.0 



64.8 



= 121. 6 



In taking thefe meafures, it was found convenient to 



make ufe of the fpace included between the two fpider's 

 lines of Troughton's micrometer, inilead of the whole field 

 of view ; which fubftitution not only prevented diilorfion, 

 by confining the objefts to the middle of the field of view, 

 but diminifhed the angle to be projedted within the dimen- 

 fions of the pupil of the eye, fo that the head did not re- 

 quire to be turned from its firft pofition in making the pro- 

 jeftion. Two painted ftaves were ftuck into the ground 

 at about 700 feet from the eye, at fuch a diftance from each 

 other, that they could both be fecn within about one -half of 

 the field of view ; the fpider's lines were then opened till they 

 coincided with the two ftaves, when projefted upon them by 

 falfe vifion, and the value of the revolutions was then 

 found to be as above ftated. This was done with the tele- 

 fcope of 30. 1 5 inches focus ; but the fame projeftion would 

 have taken place at the fame opening of the hues, with any 

 of the other three telefcopes at the fame diftance from the 

 ftaves ; therefore it was not neceflary to repeat this opera- 

 tion with the other telefcopes, becaufe the refpe£live values 

 of the fame opening, or number of revolutions, are given in 

 our Tables I. and II., as will be feen prefently, for all the 

 telefcopes, as fo many divifors for the common dividend. 

 Thus Troughton's micrometer may be ufed with great ad- 

 vantage in determining the power of any telefcope to which 

 it is adapted ; and even Cavallo's may be fubftituted for 

 the fame purpofe, when that eye-piece is ufed to which it 

 ii appropnated. But the moft convenient, as well as moft ac- 



curate dynameter that we have feen, is tiiat wliich has an eye- 

 glafs divided, fo as to form two images of the luminous dii'c, 

 when the centres of the femi-lenfes are feparated by a fcrew 

 with a divided head. This double-image dynameter was in- 

 vented by Ramfden, probably foon after Dollond's objeft- 

 glafs micrometer was invented, and is now made by G. Dol- 

 lond, and alfo, with fome variation, by Thomas Jones, of 

 Cliaring-Crofs, who was .1 pupil of Ramfden's fchool. As 

 this elegant and ufeful little infirument has not been de- 

 fcribed, we fhall introduce a Ihort account of it in this 

 place. Fig. 12. Plate X.W^. of AJlronomical InJlrumentSy 

 reprefents the exterior appearance of Mr. Dollond's con- 

 ftrutlion, and _yf^. 14. its p'r-., when the covering plate is 

 taken off ; in both which figures the fame letters refer to 

 the fame parts. The frame tliat contains the fcrew is 

 denoted by a, and b is the interior or Hiding-tube of 

 brafs, made faft to the faid frame, having the divided lens at 

 the eye -end, near the letter of reference a mjig. 1 2 ; and c is 

 the outer tube, which is placed in contaft with the outei- 

 moft eye-glafs of the telefcope, when the luminous difc is 

 to be meafured, and admits of adjuftment of the tube b 

 to diftinft vifion of the difc : d 1% R milled head of a con- 

 cealed fcrew, which feparates the two femi-lenfes until the 

 luminous difc is feen double, with the oppofite edge of each 

 difc nearly in contaft, as in ^g. 13; e is the divided head 

 of the fcrew, or micrometrical head, with 100 divifions pro- 

 perly numbered ; andy is the fcale for indicating the num- 

 ber of revolutions of the fcrew, as the divided head e does 

 the parts of a revolution. The axis of this fcrew is made 

 faft to the frame, io as not to move from its fituation 

 while it revolves, and is of bell-metal ; it is made hol- 

 low within, and is tapped to a tlu-ead of the fame fine- 

 nefs as that of the exterior fcrew ; then a fmaUer fcrew of 

 fteel enters the tapped tube, as feen in fig. 14, and has its 

 other end pinned faft to the piece of brafs g i, at the point _f, 

 which piece carries one half of the divided lens, while the 

 other half is caiTied by a fimilar piece, I h, to which the 

 fcale_/"is alfo fcrewed faft. The foot-piece of ^ A is tapped, 

 fo as to receive the thick fcrew of the axis ; and a bent 

 fpring of metal, i h, bears againft both pieces, g I and b h, fo 

 as to keep the fcrews connefted with them free from (hake. 

 During this defcription of the concealed parts of the frame, 

 the mechanical reader will have anticipated, that when the mi- 

 crometer head, and nut d, made faft to the ax's of the thick 

 fcrew, turn together in the direftion tha-, .r.akes the figures 

 increafe, the thick fcrew will draw the :,:=ce bh, and with 

 it the fcale f, and one femi-lens, towards the nut : but as 

 the fmall fcrew of fteel is a left-handed fcrew, i. e. has its 

 thread winding in a contrary direftion, and is fail to the 

 piece g b, it will recede from the nut, and take the other 

 femi-lens in a contrary direftion, fo that the centres of the 

 femi-lens will feparate with a velocity equal to the fimi of 

 the contrary motions of the two femi-lenfes ; and as thefe 

 centres recede, the original difc will become a double difc, 

 as in_yff. 13, and may by feparation be made two difcs, 

 when the femi-lenfes are removed to their greateft diftance. 

 Hence, when the value of one revolution is known, the 

 amount of any given number of revolutions and parts is 

 had, as being multiples of that revolution. In the dyna- 

 meter before us, there is .i difc of thin horn or ivory, juft 

 -r'^th of an inch in diameter, in the fliding-piece that clofes 

 the aperture of the exterior end of tube c ; and five revolu- 

 tions of the fcrew juft divides this difc into two contiguous 

 ones, fo that each fubdivifion of the micrometer e is juft 

 j^g'ggth of an inch, and when doubled, may be put down in 

 decimal numbers. But there is another ufe of the horn 

 difc, befides that of giving a value to the micrcraeter ; it 



fo 



