MORSE. — SPECTRA OF WEHNELT. 521 



In order to reach some degree of accuracy in the comparison of wave- 

 lengths the spectra were photographed with a Rowland concave grating 

 of 1G3 cm. radius, of about 2500 lines to the cm. This grating has a 

 ruled surface of 8 X 14.5 cm. and gives a very bright spectrum indeed. 

 The dispersion is of course small, the length of the first spectrum from 

 A 3200 to X GOOO being only about 5 cm. This grating was kindly loaned 

 us by Professor Langley and has proven indispensable for the work. 



The instrumental arrangements were simple. The Wehnelt cell was 

 made of a beaker, the large electrode of platinum, lead, or aluminium, 

 and the point was a wire of the metal under investigation, or where the 

 metal was in solution as a salt, of platinum or carbon. In this latter 

 case the platinum lines or the lines of impurities in the carbons used 

 often appeared, and they were used as standards with which to compare 

 the spectrum sought. The lines of the metal forming the large plate 

 also appeared after a prolonged exposure. The image of the light about 

 the point was focussed on the slit by means of a condensing lens of 

 aperture sufficient to fill the whole of the large grating with light. 



The exposures required were long, as the light is at best weak com- 

 pared with that of the spark or arc or even that of a Geissler tube. 

 For a slit opening of 0.10 mm. the exposure in the first spectrum was 

 from one to two hours. In the third spectrum, where photographs were 

 taken for the more accurate comparison of wave-lengths, the exposure 

 reached six hours, the intensity of the light varying greatly with differ- 

 ent metals. 



The spectra were photographed on orthochromatic plates, without 

 color screen, and the range of most of the photographs is from X3200 to 

 X 6000. In some cases the photograph extends much farther into the 

 red, the line of lithium at X 6709 being, for example, clearly visible. 

 The plates were developed with Amidol, to which only a very small 

 amount of sodium sulphite and a little potassium bromide were added. 

 This developer acts slowly, but permits of prolonged development with- 

 out the production of chemical fog. It is to be highly recommended 

 for work of this kind, where under-exposure is the rule and every 

 possible detail must be obtained from the plate. 



During the exposure the light was observed frequently with a direct 

 vision spectroscope, and the various parts of the glowing gaseous 

 envelope about the point were examined in the hope of finding differen- 

 ences in the spectrum at various points. Such differences were not 

 found, the spectrum being apparently the same in all parts of the 

 envelope and remaining remarkably constant throughout the exposure. 



