ON THE KEW OBSERVATORY. 353 



It consists of a pair of sliding tubes, the outer fixed, exactly at right- 

 angles, to a little base of copper, and a shield with a slit, similar to tlie ordi- 

 nary moveable shield (o\), fitted to the upper end of the inner tube. 



In using this the copper base is made to rest upon the agate planes (at s^) 

 in lieu of the ordinary knife-edge of the magnet ; the inner sliding tube is then 

 slid up or down until the slit in the shield comes to about its proper height 

 in the plane of the ordinary moveable sliield. Its edge is now marked ex- 

 actly at that point which produces a corresponding point in its image at the 

 mouth (e'), and the instrument being removed from the agate planes, the 

 distance from the lower surface of the copper base to the point marked at 

 the edge of the slit is easily measured, and is evidently the radius required*. 



The apparatus which has now been described affords us a much more con- 

 venient means of obtaining the required values than a mirror or a collimator 

 (attached to the stirrup, &c.), since the shield-arm (6^) of the declination and 

 horizontal-force magnetographs must have the same angular motion as the 

 magnet, and it can (as formerly shown) be properly adjusted in azimuth 

 about the common axis of motion : also there is a provision at the base of 

 the vertical-force magnet-support which permits a proper horizontal adjust- 

 ment for the position of the moveable shield of that instrument. 



The first parts may be said to be modifications of contrivances which have 

 been formerly used for purposes of the same kind as those to which they are 

 applicable. (They are not quite so accurate, but a little more convenient +.) 



The Scale Board, Lens, &c. used in tabulating all the self-registered curves 

 remain as described at p. 85, Report for 1849. 



A Drawing Board with Clamps is used for securing the gelatine paper and 

 Daguerreotype plate firmly in their places for the tracing process. 



A Sliding-rule, invented by Mr. Welsh, has been constructed on the same 

 principles as that described at p. S^o, " for converting the observed changes 

 of the horizontal and vertical components of magnetic force into variation 

 of dip and total force." 



A few specimens of the earlier and latter Daguerreotype curves on their 

 silvered plates have been preserved ; also some gelatine tracings, some im- 

 pressions printed from the gelatine tracings on bank-note paper, and some 



* Great inconvenience and (probably) damage to the knife-edge would ensue from any 

 attempt to acquire this measurement by means of the shield-arm {b^), itself in position. 



t The magnifying power of the lenses of the Declination Magnetograph was, in 1846, 

 ascertained by marking with angular notches, a space of about f ths of an inch on the edge 

 of a thin lamina of brass, placed in the plane of the moveable index, and by comparing the 

 length of the image projected upon ground glass at the place of the mouth (E) of the said 

 space with the space itself (on the slip of brass). 



The above-mentioned transparent glass scale is a modification of a metallic screen provided 

 with a series of slits, formerly used (and described at p. 181, Report for 18.50) in adjustments 

 for focus, and a plate is preserved exhibiting good and equally distinct impressions of the 

 images of those slits. 



The distortion occasioned by the use of a plane in lieu of a curved surface to receive the 

 image, was, from the first, a subject of gi-eat solicitude. It was no easy task to obtain a 

 tolerably " flat field" from so large a lens-aperture as I am obhged to employ upon an object 

 much removed sometimes from the axis of the lenses and at a short distance from them, 

 whilst the lenses were required to magnify several times in the conjugate focus. 



In November 1849, Mr. Fred. Ayrton promised to procure for me from Paris some very 

 finely divided scales, for the mouth-piece, on semitransparent horn, " for reading the arcs of 

 vibration described by the magnet in its diurnal variations." 



1851. 2 A 



