Telescope. 



fo mucli refcmbles the luminous diio formed by the image 

 uf the objciit-fflals which is to be meafured, that the modf 

 ot dividing this difc by the fcrew may be iUuflratlve of the 

 mode of appUcation to the meafurement of the aftual difc 

 formed with dllliuft vifion, by the refraded rays that have 

 pafTed through the eye-tube of a telefcope. 



This iuilrumeut forms, befides, a plcafmg mierofoope not 

 only for viewing, but for meafuring too, the real dimenfions of 

 any microfcopic object ; and when applied to a nicely divided 

 fcale, it may be afcertained whether or not tlie liorn difc 

 is exaftly V.,th of an inch in diameter; viz. whether or not 

 five revolutions of the fcrew will bring the llrokcs that in- 

 elude Vd th of an inch into exaft apparent coincidence ; for 

 if not, a correftion depending on the excefs or deficiency 

 inuft be applied to all meafures of a luminous difc, that are 

 to determine the total power of any telefcope ; or otherwife 

 the two lenles of tlie eye-piece muft have their Ji/lnm-c 

 between them fo adjufted, that five revolutions will exaftly 

 meafure ^' ,th of an inch ; for as the two femi-lenfes, when 

 brought to have their centres coincident, conllitute one of the 

 two lenfes of a pofitive eye-piece, as in the pearl dynameter, 

 und as in Troughton's micrometer, we have fliewn that 

 altering the diftance between thcfe lenfes, will alter their 

 compound focus, and confequently their magnifying power, 

 on which the apparent magnitude of the luminous difc 

 depends. In ufing this inftrumeut, the eye is applied above 

 the centre ot the tube i, over a in Jig. 12, and the tube c is 

 ufed, as in the peai-1 dynameter, for adjuftment for diftinCil 

 viGon of the difcs. 



When Ramfden firft made the double-image dynameter, 

 ns now conilrudled by iVIr. Dollond, and as we have here 

 defcribed it, he found that there was fome play in the 

 fcrews after they had been in ufe for fome time, fo that they 

 would not immediately obey the direA and retrograde 

 motions of the nut J; and that the lofs thus arifing affefted 

 the meafure by the femi-lenfes, which did not move con- 

 temporaneoufly ; but in the inftrument under our examination 

 there is no fault of this kind. 



The dynameter which Ramfden confidered as on an im- 

 proved conftruftion, as it regards the imperfcftion juft 

 noticed, is now made by his pupil Thomas Jones, who, we 

 have faid, has alfo improved the pearl dynameter already 

 defcribed. Figs. 10. and II. reprefent the interior parts of 

 Thomas Jones's dynameter, which we have alfo before us ; 

 there is no frame a here, but the tube b contains the lenfes 

 of the eye-piece, of which that next the eye is divided, as in 

 DoUond's inftrument ; and the tube c is the fame, except 

 that it carries a lens i, with which the divifions on the fcale 

 y are read, when the dynameter has its pofition reverfed, 

 after the meafurement is finiihed. The nut d, and divided 

 head e, are alfo the fame as we have defcribed ; but the femi- 

 Jenfes are not fixed in fliding-pieces of metal, fuch as we 

 have defcribed ; neither is the fcrew fimilar to what we 

 have above noticed. Within the tube b is an interior 

 tube of much fmaller diameter, and nearly of equal length, 

 which is divided longitudinally into two fimilar halves, which 

 turn on feparate pivots in a gimbal, or moveable ring, within 

 the remote end of the tube i, and each femi-lens is fixed in 

 the nearer end of its own femi-tube. Thefe femi-tubes are 

 marked m and n refpeftively \n.Jig. 10, and one of the 

 pivots in the ring is at 4 the other being at the oppofite 

 end of the diameter of the ring : the extreme ends of the 

 pivots turn in the tube b. The feftion of the femi-tubes, 

 holding the femi-lenfes, is leen in Jig. 1 1 , together with the 

 micrometer head and nut. The axis of the icrcw is of bell- 

 metal, and folid : the end nearefl to the micrometer head has 

 threads of double fincnefs to the end within the tube, and 



5 



theaftion is fo ing<-niou(ly tontrived, lliat the liim-lrnhs .itf 

 moved in contrarj- dircftions by the fame fcn-w, notwith- 

 llanding the threads all incline one way. The cylindrical nut 

 g is tapped for llie finer thread; and as this nut is fcrcwcd 

 tall to the lube b, as feen in Jig. i i. more plainly, the for- 

 ward motion of this axis has its velocity guided by this fmc 

 part ot the frrew ; and the end that enters the tube, ppffes 

 again!! a llud />, made fait to the femi-tube of the femi-li-ns 

 I ; ,ind a longitudinal count erasing fpring concealed in tube 

 b, and made fall to it at the lower end, .allows the fenii-tubc 

 to recede, but prelfes it cUife to the end of the fcrew ; then 

 another llud /, made fall to the femi-tube of tne femi-lens 2, 

 is tapped for receiving the coarfer thread on the axis of the 

 fame fcrew, which thus gives a double retrograde velocity 

 to this femi-tube, compared with what it receives from the 

 pulh of the finer fcrew ; and as there are two threads in the 

 fine fcrew for one in the coarfe one, and as both are cut on 

 the fame axis, the apparent motion of the femi-lens 2, it 

 aftually the difference ot two contrary, but contemporaneous 

 motions; and thefe motions are fo flow, that five revolutions 

 are equal to Voth of an inch, and confequently the reading is 

 in decimal numbers already. Otherwife, this dynameter it 

 applied exaftly as we have above explained. 



Befides thefe dynametcrs, we have cx.Tmined a double-image 

 one by DoUoud, in which the microinetcr head was divided 

 into forty parts, and in which the ivory difc was only ,'tth of 

 an inch, fo that 2.50 revolutions meafured the difc, and a 

 doubh meafure might be obtained by making the contad of 

 the two difcs firft to the left and then to tlie right, in order 

 to make the error of zero vanifh, in which cafe half the fum 

 of the two meafures was the true meafure correfted for the 

 oppofite errors of zero, and the graduated circle or head of 

 the micrometer turned IlifF on the axis of the fcrew for ad- 

 juftment to zero. This inftrument profelTcd to have 5 x 40 

 := 200 divifions in Voth of an inch, and confequently only 

 2000 in an inch ; but on examining the value of a revolution 

 Avith a fine fcale, we found that 198 divifions meafured -Vt'' 

 of an inch : we will therefore exemplify the ufe of thit 

 inftrument, by ftiewing how the correction for the imper- 

 feftion of the fcale may be applied in a£tual praflice. In 

 the firft place, the fcrew is turned in a retrograde direftion 

 until contaft of the two difcs takes place to the left of the 

 original fingle difc ; in this fituation the 40 on the divided 

 head muft be put to zero, or the lozenge marked as a 

 pointer to the micrometer head, and the ftroke indicated on 

 the fmall fcalcy", by another lozenge or index, muft be noted ; 

 then turn the fcrew, firft till the two difcs unite in one, where 

 a fingle meafure might be taken, andtiien till they are again 

 in contaft to the right ; in which fituation, the whole diameter 

 of one difc will have crofltd the whole diameter of the other, 

 and therefore the fcrew and its parts will give a dcubla meafure 

 of the real diameter. In an aftual trial of a telefcope, this 

 double meafure was found to be two revolutions of the 

 fcrew, and 37 parts of the head, or J J of another revolution ; 

 and on an average of feveral trials, i'„th of an inch was found 

 not cxadtly equal to 200, but to 198 of the divifions of the 

 head, as we have ftated above ; then ' -^l of ,' th of an inch 

 was the double meafure of the difc ; or -, !, i v = 0'295 of an 

 inch was the fingle meafure ; and the diameter of the objeft- 



giafs being 3.24 inches, we have the power = 



.0295 



no. 1 with great correftnefs, the telefcope having been pcr- 

 vioufly adjufted for viewing the folar fpots. This was the 

 determination of the power of our telefcope of 63.5 inches 

 focal diftance, when No. 4. of the celefti;d eye-pieces was on ; 

 and in the fame way aU the other powers, cclcftial or tcrrcf- 



trial. 



