278 PHILOSOPHICAL TRANSACTIONS. [aNNOI772. 



distance of such objects as appeared within the aperture of the telescope at the 

 same time, they were not of so general use as those which are contrived not only 

 to answer the ends that the first inventors aimed at, but also to take the difference 

 of right ascension and declination of such objects as are farther asunder than 

 the telescope will take in at once, but which pass through its aperture at 

 different times. Mr. Cassini first made use of threads, intersecting one another 

 at half right angles, for determining the difference of right ascensions and decli- 

 nations of objects near the same parallel ; and this apparatus, being simple and 

 easily procured, is of very great use to such as are not provided with a 

 micrometer made according to the late improvements. But, where such a one 

 is at hand, that method, however curious, need not be made use of, the micro- 

 meter serving for the same purpose with greater exactness. It was for this reason 

 indeed that the late alteration in the form of the micrometer was made, they 

 being before not so convenient for making such sort of observations, both hairs 

 being usually moveable, and no provision being made for setting the hairs 

 parallel to the diurnal motion of the objects to be observed; both which incon- 

 veniencies are avoided in the present micrometers. 



The micrometer, as now contrived, is not only of use in measuring small 

 angles or distances, between such objects as appear within the aperture of the 

 telescope at the same time, but also in taking the difference of right ascension 

 and declination between stars and planets, &c. which in their apparent diurnal 

 motion follow one another through the telescope, if kept in the same situation. 

 In making the first kind of observations, turn the short tube which carries the 

 eye glass and microrpeter, &c. till the cross thread (or that which cuts the 

 parallel threads at right angles) lies parallel to a line passing through the objects 

 whose distance is to be measured; and then, by raising or depressing the 

 telescope by help of the stand, bring the objects to appear on or near the cross 

 thread, and one of them just to touch the fixed parallel thread: then turn the 

 index of the micrometer till the moveable thread touches the other object, and 

 the number of revolutions, and parts of a revolution, shown by the index, 

 turned into minutes and seconds by the table made as hereafter directed, will be 

 the apparent angular distance of those objects. It is here supposed, that the 

 threads exactly close, so as to touch each other when the index stands at the 

 beginning of the divisions: for, if they do not, there must be an allowance 

 made in every observation; to avoid which, it is always best to adjust the threads 

 to the beginning of the divisions when they are first put on ; for which purpose, the 

 holes in the little plate which carries the moveable thread are made oblong, to give 

 room to move it as occasion requires, before it is pinched hard by the small 

 screws which fasten it to the moveable arm, through which the long screw 

 passes. The other parallel thread, which he calls the fixed one, must be first 



