204 report — 1859. 



The semicircular mirrors must likewise be placed so that their centre 

 shall be on a level with the centre of the lens. The distance from the lens 

 to the centre of the mirror is 8 - l inches. Having been reflected by the 

 mirror, the light passes through a zinc tube fixed to a slate, which connects 

 the declination pillar with the central pillar (see Plate 3. fig. 2), and so 

 reaches the hemicylindrical lens and sensitive paper already described. The 

 distance from the centre of the mirror to the sensitive paper is 6| feet. 

 Hence we have 



Distance between lens and mirror = 8*1 inches. 



Distance between mirror and cylinder . . =78 - inches. 



Total distance between lens and cylinder =86*1 inches. 



And since the distance between the slit and the lens is 17'7 inches, we find 

 that the focal distance of the lens for parallel actinic rays is nearly 14"7 inches*. 

 Before falling on the sensitive paper, the light passes through a hemicylin- 

 drical plano-convex lens (see Plate 3. fig. 1). The radius of the second sur- 

 face of this lens, is about 0*6 inch, and consequently the distance between 

 this surface and the sensitive paper (in order that the latter may be in focus) 

 is nearly P2 inch. 



2. Horizontal-force Magnetograph. 



This instrument is exhibited in Plate 4. figs. 1 & 2. The magnet, mirror, 

 lens, shade, adjustments of light and slit, &c., are in all respects similar to 

 those of the declination magnetograph already described. The peculiarity 

 of the instrument consists in the mode of suspension. A grooved wheel, £ 

 (Plate 4. figs. 1 & 2), about 0*3 in. in diameter, has its axle attached to the 

 stirrup which carries the magnet, the plane of the wheel being in the direc- 

 tion of the magnet's length. 



The suspension thread, consisting of steel wire (steel being considered 

 little liable to stretch), is carried round the wheel, and the two ends fixed to 

 the suspension roller A (see also fig. 13). A little below the suspension 

 roller the two threads pass over a screw at B, the screw being right-handed 

 where it meets the one thread, and left-handed where it meets the other. 

 Consequently by turning the screw-head, we can vary the distance between 

 the wires until it becomes equal to the diameter of the wheel, and the wires 

 will now be at the same distance from one another throughout their entire 

 length. Let us suppose that the magnet is in the direction of the magnetic 

 meridian. Turn round the torsion circle C (precisely similar to that already 

 described) until the magnet assumes a position at right angles to the magnetic 

 meridian. It is clear that, in order to do this, we shall have to turn the torsion 

 circle through an angle greater than 90°, and consequently that the plane of 

 the wires at their lower extremity will be different from that at their upper. 

 This difference is at present =35° 56' nearly. The suspension thread is 

 about 11 "6 inches long. 



As the light which falls upon the mirror in the direction de (see Plate 3. 

 fig. 1) must be reflected in the direction ef (def being 30° as before), it 

 follows that the plane of the mirror must make an angle of 75° with the 

 magnetic axis of the magnet. 



The distance between the slit and the lens is 17*7 inches, and that between 



* The focal length of the lens is determined rather by convenience of shape of the instru- 

 ment than by optical considerations. In the declination magnetograph, for instance, if the 

 distance between the slit and the lens were much greater than 1 7"7 inches, the light, bull's- 

 eye, and slit could not well be supported by an arm of the slate which is attached to the 

 declination pillar, but would require a separate piilar for themselves. 



