'266 Trowbridge and Hutchins — Oxygen in the Sun. 



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 cut out, and the shutter moved to cover 

 that part of the plate already exposed, and the lower half ex- 

 posed. The spark is then started and worked from 15 to 30 

 minutes. In addition to the spectrum of lines there is a con- 

 siderable continuous spectrum, which after a time causes fog- 

 ging 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, etc., 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 determining 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 contemplated a de- 

 termination of 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 labor and difficulty. The best of en- 

 graved 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 labor, it is difficult certainly to 

 recognize 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 comparison 

 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 : — 



