Royal Astronomical Society, 301 



The advantages peculiar to this observation are, that it is not 

 affected by ordinary refraction ; and that its scale, being one of 

 time, is exact to a degree which is unapproachable in other ways. 

 Bad, indeed, must the clock be of whose rate we are not certain 

 within 1* per diem ; therefore the uncertainty on our time-scale 

 cannot practically amount to -g-o^o-Q part of the whole, and that on 

 the results of the observation, as depends on this cause, cannot 

 amount to ^ ^ ^ ^ y part of the whole. But the Astronomer Royal 

 stated, as the result of his own experience, that an accuracy of -^^qq 

 in the determination of a micrometer-scale is almost more than can 

 be hoped for. Other advantages which it possesses are common to 

 other reversible instruments. Thus if there be a constant optical 

 fault in the image of a star, produced by defects in the object-glass, 

 that fault will produce opposite effects in the first and second posi- 

 tions of the instrument : if the pivots be mis-shapen, as if there be a 

 hump upon one, yet if the form of the two Y's is similar, that hump 

 will take the same bearing upon the east side of one Y in one ob- 

 servation as upon the west side of the other Y in another observa- 

 tion, and its effects will be annihilated in the result : if the pivots 

 are unequal, the effects of the inequality are similarly annihilated. 

 But to secure all these advantages, the two following instrumental 

 points must be secured in the construction : the instrument must 

 admit of having the level applied to it while the telescope is in the 

 position of observation ; and it must admit of being reversed with 

 ease and rapidity. To the obtaining of these objects, the construc- 

 tion devised by M. Struve, and carried out by Messrs. Repsold, is 

 particularly well adapted. This instrument is supported upon Y's 

 carried by two stone pillars, about 6 feet high and 46 inches apart, 

 from outside to outside ; the outside face of the pillars being vertical, 

 and the inside faces inclined to the vertical. The axis of the instru- 

 ment has its bearings upon the two Y's ; but the telescope (7 feet 

 long) attached to this axis is on the outside of one of the pillars ; a 

 counterpoise at the other end of the axis being on the outside of the 

 other pillar. Between the two pillars is the reversing apparatus, 

 which also carries the ordinary counterpoises. It consists of a ver- 

 tical shaft, sliding through holes in cross-bars which are fixed to the 

 piers, and prevented by a fillet upon it from turning until it is raised 

 to a certain height ; this vertical shaft carries a T head about 33 

 inches long, at the extremities of which are the lifting-forks, and 

 also the fulcra of the ordinary counterpoises. 'I'he counterpoises 

 act by means of levers to support a bar about 41 inches long, at the 

 extremities of which are the friction-rollers, which at all times sup- 

 port the principal part of the weight of the instrument. The vertical 

 shaft does, therefore, at all times support the fulcra pressures of the 

 counterpoises ; and when the instrument is raised for reversion, by 

 bringing up the vertical shaft so that the lifting-forks at the ends of 

 the T head come in contact with the axis of the instrument, the 

 only additional load which is put upon the vertical shaft is that pres- 

 sure which was left as residual weight upon the Y's on the stone 

 pillars, a pressure which, in the practice of the German astronomers. 



