404 Dr. C. Chree. Collimator Magnets and the 



In the accompanying figure, which is not drawn to scale, W and E 

 represent the " west " and " east " ends of the graduated bar on which 



D03.£ 



N ' ■' » ■ S 



^ pos.i 



DOS. 3. 



N S 



W 



DOS. 4 



S— ~N 



W 



slides the frame carrying the collimator magnet. The graduations run 

 in both directions from 0, the centre of the bar. The centre of the 

 mirror magnet should be on the vertical line through O, and also on 

 the horizontal line which is the prolongation of the magnetic axis of the 

 collimator NS. The figure shows the collimator magnet in the four posi- 

 tions which it occupies during the deflections made at one of the two 

 distances, say 30 cm. Supposing a 30 cm. deflection to be the first of 

 the day, the four positions are assumed in the order 1, 2, 3, 4. It is 

 the rule at Kew Observatory that if a 30 cm. observation takes pre- 

 cedence on a given occasion, then a 40 cm. observation has precedence 

 on the next. On the second occasion the four 30 cm. positions occur 

 in the order 2, 1, 4, 3. The object is very probably to eliminate so far 

 as possible the influence of changes of force or temperature on the 

 value of P, which is calculated from a large number of observations. 



§ 40. If there were perfect symmetry one should have identical read- 

 ings in positions 1 and 4, and again in positions 2 and 3. But, in 

 reality, agreement in these readings is Avholly exceptional. To elimi- 

 nate the disturbing influence of observational errors and changes in 

 force a number of deflection experiments are necessary. Thus, the 

 only case in which I have had the data requisite for an exhaustive study 

 is that of the Kew unifilar itself. 



In this instance the phenomena seem adequately explained by the 

 hypothesis of the existence of such asymmetry as we should anticipate 

 a priori. 



It is doubtless the aim of the maker to fit the suspension tube so that 

 the centre of the mirror magnet and the centre, or zero, of the graduated 

 bar in the deflection experiment shall lie in a vertical plane perpendi- 

 cular to the bar ; but complete success is unlikely to attend his at- 

 tempts. Accordingly, my first assumption is that the centre of the 

 mirror magnet is really at a distance z, counted positive towards the 

 " east," from the vertical plane perpendicular to the bar through the 

 centre O. 



The frame carrying the collimator magnet has a fiducial mark at the 

 centre of its edge, which is set during the deflection experiment at the 



