TELESCOPE. 



trial, may as readily b<- obtained. T. Jones's conftruftion 

 is however more convenient for ufe, and is more accura-.e, 

 though it meafures only one difc, unlets the power be great 

 and confequentiy the dlfc Imall. The divided head is fixed 

 fait to the axis of the fcrew, and is divided into loo parts, 

 coo of which meafure exadly ^,th of an inch, fo that tjie 

 inch is fub^ivided into 10,000 of thele parts, and the 

 decimal numbers aie read off at once without calculation : 

 thus, when the difc is adjufted to appear fingle and wcU 

 defined, the index, which is tlie edge of the fcale /, llanda 

 at 100, or zero of the micrometer head, and the edge ot the 

 circular rim of tlie head is coincident witli the firft Itroke of 

 the fcale; but when one revolution of the fcrew has taken 

 place, the faid edge is foufld coincident with the fecond 

 llroke of the fcale, and fo on, as the divided liead revolves ; 

 wlien the two difcs were brought into coiitaft, the quantity 

 indicated, as feen through a lens /-, was z.95, viz. two re- 

 volutions, and I'j'c on the head of the fcrew ; but in this 

 inftrument, five revolutions, we have faid, are equal to /55th 

 of an inch, and therefore one revolution = -,^3, confequently 

 Tnmjo" or -0295 of an inch, is the meafure, as before ; fo that 

 all that is requiflte to do, in regillering the meafures taken 

 with this iuitrument, is to prefix a cipher to the figures 

 read off bv infpeftion, and then the decimal quantity, or 

 divifor, is had, without further calculation, for atelefcopc of 

 any aperture, either dioptric or cata-dioptric, and of any 

 conftruclion. 



In both DoUond's and T.Jones's dynametcrs the difc is 

 feen without diftorfion and without prifmatic colours, and 

 the inftrument forms a fingle microfcope of the moft ufcful 

 kind ; for, by the latter in particular, fmall objefts may have 

 their dimenfions taken to the accuracy of ^-^i^th part of 

 an inch, and at the fame time the figures may be had by 

 infpeftion, from the fcale and its parts, to form places in 

 decimals when a cipher is prefixed, as we have above ex- 

 plained. The powers of our four telefcopes, with the eye- 

 piece of Troughton's micrometer, were found by T. Jones's 

 double-image dynameter agreeably to the fubjoiued ftate- 

 ment ; viz. 



In. 

 30.15 focus 



[ '•50 ^ 

 1 .040 



45-75 



ditto 



•50 

 .049 



3-5° 



^0.6 =: power. 



63.5 ditto 



118.8 ditto 





076 

 3-25 



= 46.0 = ditto. 



0504 

 000 



64.5 



ditto. 



0252 



=^ 1 19.0 := ditto. 



In all the three determinations of powers, the adjuftment 

 for vifion was to a diftant terrejlrial objeft, and confequently 

 thcfe are fomewhat too great. From the experience we have 

 had of thefe different modes of afcertaining the powers of a 

 telefcope, we have no hcfitation in giving the preference to 

 the double -image dynameter, in which the two images may 

 be brought into very nice contaft : whereas in the pearl 

 dynameter, which is alfo very good, fomething is always left 

 to eftimation in taking the fractional part of a divifion ; and 

 wheafalfe vifion is ufed, the adjuilmentof the eyes to differ- 

 ent diftances, one within the telefcope and the other with- 

 out, at the fame time, leaves confiderable uncertainty in the 

 fize of the projefted field of view, which will vary according 

 to the flate and pofition of the eye in every trial. We 

 aierition this laft circumftance as worthy of confideration, 

 becaufe feveral micrometrical determinations of the diftance 

 between double ftars have been made, particularly by fir W. 



Herfchel, where the power of the telefcope determined by 

 falfe vifion is made in effeft the fcale of the meafure ; confe- 

 quently if the power is not accurately afligned, the meafure 

 of the angular diftance depending on it will be proportion- 

 ably erroneous. Aftronomers, we repeat, are yet in 

 want of an unobjectionable mode of meafuring the angu- 

 lar diftances of very fmall double ftars, which cannot be leen 

 when extraneous light is admitted into the telefcope, and 

 which therefore have hitherto been projefted on two lumi- 

 nous points, placed at meafured diftances from each other, 

 for the purpofe of afcertaining the apparent celeftial interval 

 in its magnified ftate, from a comparifon with a known ter- 

 reftrial interval in its unmagnified ftate, which method is 

 liable to confiderable uncertainty, and can only be admif- 

 fible upon the principle of its admitting of an average 

 taken from a fucceffion of meafures under different circum- 

 ftances. 



We have already explained, in our preceding feftion, how 

 the powers of any telefcope that has a terreftrial eye-tube, may 

 be varied by the application of the celeftial eye-pieces to the 

 eye-end of this, by the help of adapters, and therefore we 

 fliall only fay further on this part of our fubjeft, that how- 

 ever the power is varied by changes of pofition of the eye- 

 pieces, or by additional field-glaffes, any of the dynameters 

 will give the total power, under any of the difpofitions, by 

 the fimple nieafurement of the difc, which we have explained ; 

 but (liould there be any doubt about the exclufion of the rays 

 incident on the extreme circular edge of the objeft-glafs by 

 the diaphragm, or by Troughton's new illuminator, a mea- 

 fured circle, or long flip of paper, ftuck to the face of the 

 objeft-glafs, muft neceffarily be fubftituted for the glafs itfelf, 

 which we were obliged to do with three out of the four of 

 our telefcopes, and then its image at the eye muft be fubfti- 

 tuted for the difc, that we have hitherto defcribed as the true 

 image of the glafs itfelf, which it will be only when all the 

 rays are tranfmitted and refrafted to a focus at the place of 

 the faid ditc or image. 



In all refrafting telefcopes, that are not achromatic, of 

 which indeed very few are now made, the indiflindnefs of an 

 objeft is direftly as the area of the aperture, and inverfely 

 as the fquare of the focal diftance of the eye-glafs, when this 

 is fingle, becaufe the aberrations are proportional to thefe 

 data ; but in a reflefting telefcope, the indiftinftnefs will be, 

 with fpherical curves, as the fixth power of the diameter of 

 the large fpeculum direftly, and as the fourth power of its 

 focal diftance inverfely, and alfo as the fquare of the focal 

 diftance of the eye-glals inverfely. 



The light in any telefcope, refrafting or reflefting, if we 

 difregard what is loft by refleftion, is direftly as the fquares 

 of the linear aipertures, and inverfely as the fquare of their 

 linear amplifications. 



In refrafting telefcopes of various lengths, not achromatic, 

 a given objeft will appear equally bright and diftinft, when 

 their linear apertures, and the focal diftances of their fingle 

 eye-glaffes, are feverally in a fubduphcate ratio of their 

 lengths, or focal diftances of their objeft-glaffes : and then 

 alfo their linear amphfications will be in a fubduphcate ratio 

 of their focal lengths. But in reflefting telefcopes, and in 

 the beft achromatic refraftors, of various lengths, a given 

 objeft will appear equally bright and equally diftinft, when 

 their linear apertures, and alfo their linear amplifications, are 

 as the fquare-fquare roots of the cubes of their lengths ; and 

 confequently when the focal diftances of their eye-glaffes 

 are alfo as the fquare-fquare roots of their lengths. See 

 Smith's Optics, p. 140, et feq. 



6. Meafures taken by MicrometriealTelefcopes. — Though the 

 primary ufe of a telefcope is to render a diftant objeft 



vjfible, 



