Oxygen in the Sun. 305 



of five feet focus. The method of working with the apparatus 

 so arranged has been as follows. 



The points of aluminium being permanently fixed in front 

 of the slit, sunlight is introduced, the camera brought to focus 

 once for all, and set to any required wave-length upon a con- 

 venient scale. The photographic plate is then placed in the 

 camera, and a shutter immediately in front is set to expose 

 the upper half of the plate. Exposure for the sun is then 

 made ; the sunlight is then shut out, and the shutter moved to 

 cover that part of the plate already exposed, and the lower 

 half exposed. The spark is then started and worked from 15 

 to 30 minutes. In addition to the spectrum of lines there is 

 a considerable continuous spectrum, which after a time causes 

 fogging of the plates ; so there does not seem to be any gain 

 in an exposure of more than half an hour. The feebleness of 

 the air-lines can be judged of when we state that, with the 

 same plate, breadth of slit, &c, we get a metallic spectrum in 

 the arc in ten seconds, strongly photographed. There was 

 sufficient iron present in the electrodes as impurity to give the 

 strongest iron-lines feebly, and these have been of use in deter- 

 mining that no displacement had happened, although, from the 

 nature of the arrangements, such disturbance could hardly occur. 



On the negative produced as above indicated the two 

 spectra lie exactly edge to edge, like a vernier and scale, and 

 are in the best possible position for the accurate determination 

 of the position of the air-lines. The original plan contem- 

 plated a determination of the wave-lengths of all the air-lines 

 throughout the entire spectrum ; but persistently bad weather 

 and other causes have compelled the postponement of the 

 completion of this work, though we are now able to give it 

 complete from wave-length 3740 to wave-length 5030. 



The photographic map of the solar spectrum of Professor 

 Rowland has made easy what would otherwise have been an 

 undertaking of extreme labour and difficulty. The best of 

 engraved maps of the violet region of the spectrum to beyond 

 F are comparatively worthless. Even on the elaborate map 

 of Vogel, the result of years of labour, it is difficult to recog- 

 nize with certainty other than the more prominent lines, and 

 you never feel quite sure of your positions ; but we turn to the 

 map of Rowland with the certainty of finding every line in 

 its true order and magnitude, so that what was formerly most 

 difficult has now become very simple, and the position of any 

 well-defined air or metallic line can be read directly, by com- 

 parison of the photograph with the map, "to the tenth of a 

 wave-length. 



We here give a table of wave-lengths as determined from 

 our photograph of the sun and air spectra : — 



