296 



Sir W. Crookes. 



[Aug. 1, 



The spectrograph itself is a five-prism quartz instrument which I 

 have had in use since 1894. A general idea can be gained of the 

 arrangement of the spectrograph from the accompanying outline plan 

 (fig. 1). The light from the source, a, passes first through the 

 condenser, b, on to a slit, c, thence to the collimating lens, d, and 



FIG. 1. 



round the train of prisms, e. It then passes through the camera 

 lens, /, whence it falls on the sensitive film in the holder, g. The 

 whole apparatus is mounted on a planed cast-iron table, h, i, j, k, 

 49 \ inches long, 31 inches wide, and 1J inches thick. The prisms, 

 lenses, and photographic film-holder are enclosed in a large wooden 

 framework, /, m, n, o, p, q, r, s, having aluminium shutters on each 

 side, so that the inner adjustments can be effected easily. The 

 height of the enclosure is 28 inches, and the whole is perfectly light- 

 tight when the shutters are down. Outside the dark enclosure, in 

 the space A, I, s, r, are situated the arrangements for producing the 

 light to be examined, the optical condensers, and the slit. 



An electrical condenser of 180 square inches is intercalated in the 

 secondary circuit, and a coil of twelve turns of well-insulated wire, 

 each turn being 12 inches in diameter, is interposed in the path of 

 the secondary current. This introduction of self-induction in the 

 circuit suppresses most of the air lines, and gives the metal lines on a 

 black background. 



The optical efficiency of a spectrograph depends in no small degree 

 on the way in which the source of light is presented to the slit. If 

 the electrodes are too far apart there is distortion, owing to each pole 

 forming its own luminous centre, and if the spark is close to the 

 slit we have to contend with other irregularities; the image is not 

 clear, and the lines are often confused and blurred. If the spark is 



