ON SELF-RECORDING MAGNETOGRAPHS. 203 



means of a water-regulator. The burner consists of a narrow slit about three- 

 quarters of an inch long, and one-hundredth of an inch in breadth. It is 

 placed endwise with respect to the lens, in consequence of which position, 

 the light (coming from a stratum of name three-quarters of an inch in depth) 

 has its brilliancy greatly increased (see Plate 4. fig. 10 a). 



The shape of the burner and the arrangement for supplying the flame 

 with air, are in all respects similar to those used in a paraffin lamp, their 

 application to gas having been suggested by Mr. Beckley. The burner is 

 fitted with a glass chimney, the presence of which intensifies the light — it 

 must not, however, fit too tightly. 



The bull's-eye lens used for condensing the light of the gas upon the slit 

 is that known as the double condenser. 



Having passed the bull's-eye lens, the light falls upon the slit c. The 

 breadth of this slit is about y-J^th of an inch ; a front view of it is given in 

 Plate 4. fig. 10 a. 



By means of an adjustment, the distance between the gas-flame and the 

 bull's-eye lens may be altered until the slit is in focus for the gas-flame. 



The light having passed the slit, goes through a covered tube until it 

 reaches the plano-convex achromatic lens before mentioned. By means of 

 an adjustment, the gas-flame, the bull's-eye, and the slit may be moved 

 together until the slit be at that distance from the lens which is the conju- 

 gate focus of the sensitive paper. There is also an arrangement by which 

 gas, bull's-eye, and slit may be moved a little to one side of the central line 

 of the lens, so that the two dots may be made to assume a different position 

 on the sensitive paper. 



The distance between the slit and the lens is 17'7 inches. This lens is fitted 

 into a glass shade which covers the magnet, as represented in Plate 4. fig. 2. 



This glass shade stands upon a circular marble slab, diameter 20 inches, 

 thickness 1*2 inch, which is cemented to the top of a solid pillar of Portland 

 stone 4 feet high. 



There are two holes cut in this glass shade, each about 3 inches in diameter 

 (see Plate 4. figs. 1 & 6), the one to contain the lens above mentioned, 

 through which the rays of light pass on their way from the slit to the mirror ; 

 and the other to contain a piece of plane glass through which the same rays 

 pass on their way from the mirror to the cylinder. The glass shade is gilded 

 inside nearly to the top. This gilding serves the double purpose of reflect- 

 ing back any heat associated with light which may strike it from the outside, 

 and (being a bad radiator) of diminishing as much as possible the currents of 

 air which changes of temperature are apt to produce. The portion of the 

 shade which is not gilded is covered outside with a cloth cap, removeable at 

 pleasure. A vessel containing chloride of calcium is put inside to absorb all 

 moisture. A curved arm of brass (Plate 4. figs. 3 & 4) carries the suspen- 

 sion roller A, and torsion circle C (see also fig. 14) reading to minutes. The 

 suspension thread is a silk fibre slightly rubbed with bees-wax, in order to 

 render it less susceptible to hygrometric influences. 



The magnet (D) is a rectangular bar about 5'4 inches long, 0*8 inch 

 broad, and 01 inch thick. The semicircular mirrors, already alluded to, 

 are also represented in figs. 3 & 4. Their diameter is 3 inches ; and great 

 care has been taken that the glass surfaces should be accurately plane and 

 parallel to each other. G is a copper damper, the object of which is to check 

 the oscillations of the magnet, and bring it to rest speedily. The angle aef 

 (Plate 3. fig. 1) being =30° and ef being perpendicular to the magnetic 

 meridian, it follows that the plane of the mirror must be inclined at an angle 

 of 15° to the axis of the magnet, in order that the ray de may be reflected in 

 the direction ef 



