MICROMETER. 



205 



teri. 



Ki. 1*. 



Double even known to Mr. Servington Savary of Exeter, in 

 Image the year 1743, when he communicated to the Royal 

 ** r;rom * > Society of London an account of a double image micro- 

 meter, consisting of two lenses, capable of being sepa- 

 rated from.and of being brought near to, one another by 

 mechanical means. When the two images of the sun, or 

 of any other body, were in contact, the distance of the 

 lenses becomes a measure of the angular magnitude of 

 the sun, the value of the scale having been previously 

 determined by experiment. The account of Savary 's 

 micrometer was extracted from the minutes of the So- 

 ciety by Mr. Short, and published in the Phil. Trans. 

 for 1753. In 1748, M. Bouguer proposed a similar 

 instrument, which he called a heliometer ; and in 

 Mr. Dollond made a farther improvement on this kind 

 of micrometer, by substituting in place of two object 

 glasses single object glass cut in two. As a drawing 

 and description of Mr. J. Dollond's divided object glass 

 micrometer has already been given in our article AS- 

 TRONOMY, vol. li. p. 734, and in Plate XL VI II. Fig.4 and 

 5, it is unnecessary to enlarge the present article by any 

 description of the more imperfect instruments of Savary 

 and Bouguer. We shall content c-inelves with ex- 

 plaining the general principle of all instruments of this 

 kind. 



General Let the two semi-lenses have their centres at E and 



principte of H, end their principal focus at F, and let P, Q, be 

 divided ob- two distinct objects, or the opposite limits of the same 

 jet f ! object lying in FE, FH prolonged. The images of the 

 twoobjec rmrdby each semilens, will coin- 



i" 4 TE cide at F, and therefore two images of the circular ob- 

 ccctxxv. jert PBQD, vi. : F, pxf, will be found touching 

 one another at F. Hence the angular measure 

 will be equal to the angle which I H or the distance of 

 the centres of the semilenses forms at F, and, as the 

 angles measured are very small, they will always vary 

 as EH. The angle, therefore, which corresponds to 

 any one distance of the centres , H, being known by 

 the methods already described, the angle 

 ing to any other distance will be obtained by 

 proportion Mr. Dollond first adapted his micrometer 

 to the object end of a reflecting telescope ; but his son 

 afterwards applied it to achromatic telescopes. 



The divided object glass micrometer has a great su- 

 periority over the wire micrometer, in so far as it enables 

 us to measure sny diameter of the sun or moon, whatever 

 be its inclination to the direction o.f their motion ; 

 whereas in the wire micrometer, we can only measure 

 that diameter which is perpendicular to the direction 

 of the motion. 



Apptirukm In order to apply the divided object glass) microme- 

 io the me- let to determine differences of right ascension and de- 

 of clinstion, Dr. Maskelyne has pointed out the following 

 ~ ' method. 



" Let H C R c, Fig. 15, be the field of view, HR and C c 

 two wires bisecting the field of view at right angles to one 

 another, and having a motion in their own plane, turn 

 the wires till the westernmost star (which is the best, 

 having further to move) run along ROH ; then sepa- 

 Fif. IS. rate the two segments, and turn about the micrometer 

 till the two images of the same star lie in the wire C e ; 

 and then, partly by separating the segments, and part- 

 ly by raising or depressing the telescope, bring the two 

 innermost images of the two stars to appear and run 

 along ROH, as a, b, and the vernier will give the dif- 

 ference of their declinations ; because, as the two im- 

 ages of one of the stars coincided with C c, the image 

 off each star was brought perpendicularly upon HR, or 

 to HR in their proper meridian. And, for the same 



reason, the difference of their times of passing the wire 

 CO c will give their difference of right ascensions. 

 These operations will be facilitated, if the telescope be 

 mounted on a polar axis. If two other wires KL, MN, 

 parallel to C c, be placed near r} and R, the observa- 

 tion may be made on two stars, whose difference of me- 

 ridians is nearly equal to HR, the diameter of the field 

 of view, by bringing the two images of one of the stars 

 to coincide with one of these wires. If two stars be 

 observed, whose difference of declinations is well set- 

 tled, the scale of the micrometer will be known. 



It has hitherto been supposed, that the images of the 

 two stars can be both brought into the field of view at 

 once upon the wire HOR ; but if they cannot, set the 

 micrometer to the difference of their declinations as 

 nearly as you can, and make the image which comes 

 fir.-t run along the wire HOR, by elevating or depress- 

 ing the telescope ; and when the other star comes in, 

 if it does not also run along HOR, alter the micrometer 

 till it does* and half the sum of the numbers shown by 

 the micrometer at the two separate observations of the 

 two stars on the wire HOR will be the difference of 

 their declinations. That this should be true, it is ma- 

 nifestly necessary that the two segments should recede 

 equally in opposite directions ; and this is effected by 

 Mr. Dollond in his new improvement of the object 

 glass) micrometer. 



The difference of right ascensions and declinations 

 of Venus or Mercury in the sun's ditk and the sun's 

 limb may be thus found. Turn the wire*, so that the 

 north limb ii. Fig. 1 6, of the sun's image AB, or the north Fig. 1C. 

 limb of the image V of the planet, may run along the 

 wire RH, which therefore will then be parallel to the 

 equator, and consequently Com secondary to it ; then 

 separate the segments, and turn about the micrometer 

 till the two images V, f, of the planet pass C cat the 

 same time, and then by separating the segments, bring 

 the north limb of the northernmost image V of the 

 plsnet to touch HR, at the time the northernmost limb 

 n of the southernmost image AB of the sun touches it, 

 and the micrometer shows the difference of declinations 

 of the northernmost limbs of the planet and sun, for 

 the reason formerly given, we having brought the 

 northernmost limbs of the two innermost images V 

 and AH to HR, these two being manifestly interior to 

 v and the northernmost limb N of the image 1'Q. In 

 the same manner, we take the difference of declinations 

 <it their southernmost limbs ; and half the difference of 

 the two measures (taking immediately one after ano- 

 ther) is equal to the difference of the declinations of 

 their centres, without any regard to the sun's or pla- 

 net's diameters, or error of adjustment of the microme- 

 ter ; for as it affects both equally, the difference is the 

 same as if there were no error ; and the difference of 

 the times of the transits of the eastern or western limbs 

 of the sun and planet over C c gives the difference of 

 their right ascensions. 



Instead of the difference of right ascensions, the dis- 

 tance of the planet from the sun's limb, in lines paral- 

 lel to the equator, may be more accurately observed 

 thus : Separate the segments, and turn about the wires 

 and micrometer, so as to mske both images V, v, Fig. 17, pig. 17. 

 run along HR, or so that the two intersections I, T of the 

 sun's image may pass C c at the same time. Then bring 

 the planet's and sun's limbs into contact, as at V, and 

 do the same for the other limb of the sun, and half 

 the difference gives the distance of the centre of the 

 planet from the middle of the chord on the sun's disk 

 parallel to the equator, or the difference of the right 



