Photography at Greenwich Observatory. 19 



for the finest cobweb would have effectually fettered the deli- 

 cate motions of these instruments, and therefore photography 

 was at once looked to for the required assistance, and to the 

 inventive talents of Charles Brooke,, Esq., F.R.S., the eminent 

 surgeon and microscopist,. we are indebted for the plan adopted 

 about the year 1846, and which has since been in constant 

 operation for the registration of the magnetic instruments 

 we have described, as well as the barometer and thermo- 

 meters. 



The photographic process employed is a paper one, being 

 a modification of the calotype, in which an invisible trace, 

 produced by the action of light, is subsequently developed by 

 suitable re-agents, and, waiving for the present all details of 

 its preparation, we will first consider the mechanical arrange- 

 ments of the apparatus. The sheets of paper, when properly 

 sensitized, are wrapped round glass cylinders, which are, in 

 fact, the ordinary glass shades used to protect works of art. 

 Those used are 11| inches long and 14|- inches round. Each 

 shade is cemented into a brass cap, having a spindle projecting 

 from the centre. This pin and the hemispherical end of the 

 cylinder rest on friction rollers in the case of a horizontal 

 cylinder, and a wire from the spindle, bent into the form of a 

 winch, rests in a hole in the hour-hand of a strong time-piece, 

 by which it is made to rotate smoothly in twenty-four hours. 

 In the case of a vertical cylinder the time-piece is placed below 

 the glass shade, the arrangement for rotating it on its axis 

 being the same. 



After the prepared paper is rolled round the cylinder, 

 it is covered by another shade slightly larger, and the two are 

 made to fit by wet tape round the mouth, and some damp 

 wadding is also placed in the spherical end of the shade 

 to keep the paper moist. The registration of the magnets is 

 effected by allowing a beam of light from a very narrow slit in 

 the copper chimney of a gas-flame placed a little out of the 

 line joining the magnet and the revolving cylinder, to fall on 

 a concave mirror carried by a part of the apparatus, suspending 

 the magnet, and, of course, moving with it. This causes the 

 beam of light to converge nearly on the centre of the cylinder 

 about twelve feet off, where it falls upon a plano-convex cylin- 

 drical glass lens, having its axis parallel with the axis of the 

 cylinder, by which the image of the slit is reduced to a neat 

 spot or pencil of light. The magnet moving in azimuth, pro- 

 duces a spot of strong light, which runs along this lens, and, 

 as the paper revolves under it, this light, by its actinic power, 

 produces a continuous line round the cylinder, deviating to the 

 right or left, and thus indicating the horizontal motions of the 

 magnet. 



