530 



Major J. Herschel. 



explained, although unimportant in absolute magnitude, are all- 

 important relatively. The prime defect of the instrument is to be 

 seen in the insufficient optical means of noting and recording this 

 relation. This might be remedied without altering the instrument in 

 any way, by changing the collimator ; so I will not dwell further on 

 that, but pass on to the means of controlling and adjusting this 

 angular position. Inasmuch as torsion of wires is in question, it is 

 obvious that the way in wMeh their ends are held and turned is a detail 

 of extreme importance. Mr. Broun has alluded to this. He says 

 they " are fixed at their ends in a special manner, so that the fixed 

 points cannot vary." It is unfortunate that one cannot learn cer- 

 tainly by inspection, what the attachment is. It is probably by the 

 pinch of a split screw,, for the wires appear to pass through the axes 

 of the holding screws. Whatever the method, is, it ought to be un- 

 impeachable. But not only should the holding be secure, it should 

 also be easily manageable. It is of little avail to attach a fine wire to 

 a delicate screw which can only be manipulated with caution by a 

 steady hand, for the adjustments depend mainly on these ends of 

 wires being turned accurately through very small angles. I regard it 

 as a capital error of construction that the grasp of the ends of these 

 suspension wires is made as small instead of as large as possible. 

 This is an opinion based on wearisome experience no less than on 

 common sense, for I have spent many hours in endeavouring to obtain 

 the adjustment in question, with no other result than this experience 

 and the discovery of the cause of repeated failure — as I will now 

 explain. 



The first adjustment required would seem to be to. make the minor 

 weight hang so that its mirror shall be parallel to the middle mirror 

 of the major weight. To secure this the holding of the single sus- 

 pending wire must be turned, either above or below. It is difficult, if 

 not impossible, to get at the lower holding. But the upper one offers 

 no difficulty except what is to be expected from the smallness of 

 the parts. The necessary adjustment was at length made, approxi- 

 mately. 



The second adjustment consists in so managing the torsion of the 

 double wires, that when the minor weight (with its mirror) is turned 

 through 360° either way, the major weight shall present its. right or 

 its left hand mirror equally short of or beyond the ultimate position. 

 The observation in that case will consist in ascertaining what alteration 

 of weight will bring about exact conjunction, in either case. 



The difficulty, which I have already commented on, of giving any 

 precise amount of rotation to. the holding screws, is in this matter 

 also so great as to make exact adjustment quite fortuitous. This will 

 explain why I made the same adjustment several times in succession 

 (on each occasion with sorely tried temper and patience) before 



