280 PHILOSOPHICAL TRANSACTIONS. [aNNO 1772. 



angle may be taken in between the parallel threads, if need be. And it must 

 always be remembered that the moveable parallel thread should be set either north 

 or south of the other, according as the following star is expected to be really 

 south or north of the preceding. 



In making an observation, either the inner or the outer edges, or the middle 

 of the wires, may be brought to touch the objects; but then it must be remem- 

 bered to allow something for the thickness of the wire, in case the observation 

 be not made from that part to which the index is adjusted. In observing the 

 diameters of the sun, moon, or planets, it may perhaps be most convenient to 

 make use of the outer edges of the threads, because they will appear most distinct 

 when quite within the limb of the planet, &c. ; but if there should be any sen- 

 sible inflection of the rays of light in passing by the wires, this would be best 

 avoided by using the inner edge of one wire and the outer edge of the other. 

 And in taking the distance or difference of declination between two stars, &c. 

 the middle of the threads may perhaps be most convenient : but, however the 

 observation is made, due correction must be allowed for the thickness of the 

 wire, if requisite. 



The difference of declination of two stars, &c. may be observed with great 

 exactness, because the motion of the stars is parallel to the threads; but in taking 

 any other distance, the motion of the stars being oblique to them, is a great 

 impediment, because if one star be brought to one thread, before the eye can 

 be directed so as to judge how the other thread agrees to the other star, the 

 former must be somewhat removed from its thread, so that in this sort of ob- 

 servations the best way of judging when the threads are at the proper distance, 

 is by frequently moving the eye backwards and forwards from one to the other : 

 this method must chiefly be made use of when the distance of the objects is 

 pretty large, and the motion or rolling of the eye great. 



The micrometer is so contrived that it may be applied to telescopes of different 

 lengths ; but then there must be a table for each telescope, by which the revolu- 

 tions of the screw may be turned into minutes and seconds of a degree. In order 

 to this, it is necessary that the threads of the micrometer should be placed 

 exactly in the common focus of the object-glass and eye-glass, that is, where 

 images of objects seen through the telescope are distinctly formed. The readiest 

 way of doing this, is, first to slide the micrometer into the grooves fixed to the 

 short brass tube, which carries the whole apparatus of eye-glass, &c. and then to 

 draw the eye-glass out by means of its sliding work, till the threads of the mi- 

 crometer be in its focus, which is known by their appearing most distinct, &c. 

 Then thrust the short tube before mentioned into its proper place, as flir as the 

 shoulders of the brass work will admit, and place the object glass in its cell, and 

 looking through the telescope at some very distant object, slide the wooden tube 



