VOL. LXI.] PHILOSOPHICAL TRANSACTIONS. 207' 



strs can be brought to appear within the field of the telescope on the wire ew at 

 the same time; but this is not absolutely necessary. For if the micrometer be 

 set to the difference of the declination nearly, and then the star which passes 

 first through the telescope be made to run along the wire ew, by touching one 

 of the handles of the ra<!k-work. of the telescope, and afterwards the other star, 

 when it comes into the telescope, be brought to the wire ew by altering the 

 opening of the glasses of the micrometer, the difference of the declination will 

 be had, by taking half the sum of the numbers shown by the micrometer, at the 

 two separate observations of the two stars on the wire ew. This will be true, 

 in case it can be depended on that the two semicircular glasses recede equally in 

 contrary directions; which may indeed be doubted, the work on which the 

 motion of the glasses depends not being designed for such a purpose, and there- 

 fore probably not made sufficiently accurate for it. 



The manner in which Mr. DoUond has contrived the motion of the glasses, 

 in his new improvement of the object glass micrometer, entirely obviates this 

 difficulty, and the difference of right ascension and declination of any two stars, 

 or other points in the heavens, may be thus accurately measured, let the 

 difference of right ascension be what it will, provided the difference of declination 

 does not exceed the extent of the scale of the micrometer; and thus the object 

 glass micrometer is put pretty much on a footing with the common micrometer, 

 even with respect to the measuring right ascensions and declinations. jm 



The difference of right ascension and declination between Venus or Mercury 

 and the sun's limb, in their transits over the sun, are to be observed nearly in the 

 same manner as the difference of right ascension and declination of two stars^ 

 But the process will perhaps be rendered clearer by the following description. 

 1. Turn the moveable wires ew, ns, into such a position, that the sun's north 

 limb n, fig. 10, or the planet's north limb v, may run along the wire ew, which 

 thus becomes a tangent to the peripheries of their disks. 2. The semicircular 

 glasses being separated to a convenient distance, turn the micrometer about, till 

 the two images of the planet v, v, pass over the horary wire ns at the same 

 instant. 3. Separate the glasses of the micrometer to such distance, that the 

 north limb v of the northernmost image of the planet may touch the wire ew, 

 at the same time that the northernmost limb n of the southernmost image of the 

 sun touches the same wire; and the scale of the micrometer will show the 

 difference of declination of the northern limbs of the sun and planet. In like 

 manner, if the glasses of the micrometer be opened to a greater or less distance, 

 according as the planet is nearer the north or south limb of the sun, every thing 

 else remaining unmoved, the difference of declination of the southern limbs of 

 the sun and planet may be observed, by bringing the southernmost limb of the 

 southernmost image of the planet to run along the wire ew, at the same time 



