206 , PHILOSOPHICAL TRANSACTIONS. [aNNO 1771. 



the meridians passing through the stars does not exceed cw, the semidiameter of 

 the field of the telescope. Turn the wires ew, ns about, till one of the stars 

 (the westernmost star will generally be best for this purpose) runs exactly along 

 the wire ew, by the diurnal motion. Then separate the 2 segments of the 

 divided object glass to a convenient distance, and turn the micrometer round 

 about, by means of the proper handle, till the two images of the same star, 

 formed by the two segments of the object glass, pass the horary wire ns at the 

 same instant. Lastly, partly by separating the glasses, and partly by touching 

 the rack-work screws of the stand of the telescope, cause the southernmost 

 image of the northernmost star, and the northernmost image of the southernmost 

 star, to appear both upon and run along the wire ew, as a, b. The numbers 

 standing on the scale of the micrometer will show the difference of declination 

 of the stars; and if the times be noted when they pass the horary wire ns, the 

 difference of the times will give the difference of their right ascension. For ew, 

 on account of the star's running along it, is parallel to the equator; and con- 

 sequently NS, which is perpendicular to it, represents a meridian or horary circle. 

 And because- the two images of the same star pass the horary wire ns at the same 

 instant, it follows that the centres of the two semicircular glasses lie in the same 

 meridian, and consequently when a, b, the two contrary images of the two stars 

 are brought to the same parallel of declination ew, the scale will show the 

 difference of their declinations. And, for the same reason, the times of the 

 images of the two stars passing the meridian wire ns will not be affected by the 

 separation of the glasses of the micrometer, and consequently the difference of 

 the times will give the difference of their right ascension. It will be easily 

 understood that in performing the operations above described, it will be necessary 

 from time to time to turn the screws of the rack-work which move the whole 

 telescope together. These operations will be much facilitated and rendered more 

 exact, if the telescope be supported by and moveable on a polar axis; for the 

 wires and micrometer may be thus more brought into the requisite positions, and 

 the turning of the telescope about in order to follow the diurnal motion will not 

 disturb those positions: which will afford this further advantage to the observer, 

 of being able to repeat the observations without loss of time. 

 ' If two additional horary wires fg, hi parallel to ns be placed near e and w, the 

 two extremities of the wire ew, the adjustment of the wires and micrometer n)ay 

 be more readily performed, and the observation may be made on two stars, 

 though their meridian distance from one another should be almost equal to ew, 

 the diameter of the field of the telescope. It is evident, that if two stars be thus 

 observed whose difference of declination is well settled, the value of the scale of 

 the micrometer may be thence determined. 



In the foregoing directions it has been supposed that the images of the two 



