260 REPORT— 1880. 



given the wave-lengths of all atmospheric lines, but has not separated 

 the oxygen and nitrogen lines. Huggins' measurements have been 

 reduced to wave-lengths by Watts (Index of Spectra). At atmospheric 

 pressure the lines are not shai-p, so that an exact measurement is difficult. 

 Pliicker and Hittoi'f have given a drawing of the lines as seen in vacuum 

 tubes with jar and air-break ; but they did not use a sufficient dispersion 

 for accurate measurement, and their points of reference are so few, that 

 the reduction to wave-length made by Watts was attended by many 

 difficulties, and the result is not altogether satisfactory. A careful set of 

 measui'ements of the nitrogen lines as they appear, when the pressure is 

 low and with high dispersive power, would be a very useful addition to 

 our knowledge of this spectrum. The continuous spectrum generally 

 accompanying this spectrum has been investigated by Wiillner (1869). 



The Band-fipectrum of the Positive Discharge. — This spectrum which is 

 generally called the ban'l-spectrum of nitrogen, always appears when the 

 discharge is sufficiently reduced in intensity. 



It was first observed by Pliicker (1858) in a vacuum tube, and about 

 the same tioae by v. d. Willigen in the brush discharge of an ordinary 

 electrical machine. The best way of obtaining it is that adopted by Pliicker, 

 who was the first to introduce the shape of vacuum-tubes now generally 

 in use with the capillary part. Hence these tubes are often called Pliicker's 

 tubes. The capillary part increasing the resistance greatly increases the 

 luminosity of the discharge. If nitrogen (or atmosphei'ic air) be intro- 

 duced into such a Pliicker tube, the capillary part will shine, on reduction 

 of pressure, with a rose-coloured light, when the ordinary discharge is 

 sent through it. The spectrum is one of the most beautiful which can be 

 observed. A \cvy good coloured drawing of it is given in Pliicker and 

 Hittorf's paper. Accurate measurements of the bands are given by Ang- 

 strom and Thalen. Another drawing with measurements will be found 

 in Lecoq de Boisbaudran's Atlas.' 



The bands of this spectrum, which are situated in the red and yellow, 

 present a different appearance from those which are seen in the blue and 

 violet. This fact has led Pliicker and Hittorf to the supposition that 

 we have to deal with two different spectra which are superposed only but 

 given out by two distinct sets of molecules. The authors tried and suc- 

 ceeded in separating the two spectra. By increasing the diameter of the 

 capillary part they obtained a tube which only showed the red and yellow 

 bands. Their experiment is described in the following words, which will 

 be found in the 28th paragTaph of the paper mentioned at the head of this 

 chapter : — 



'Thus we succeeded in constructing a tube which, when the direct 

 discharge was sent through it, became incandescent with the most brilliant 

 gold-coloured light, which might easily be confounded with the light of 

 highly-ignited vapours of sodium ; but with the intercalated jar, the light 

 of the incandescent gas within the same tube, had a fine bluish- violet 

 colour. The yellow light when analysed by the prism, gave a beautiful 

 spectrum of shaded bands, extending with decreasing intensity to the 

 blue, the channelled spaces being scarcely perceptible. The bluish light 

 when examined was resolved by the prism into channelled spaces, extend- 

 ing towards the red ; while the former bands almost entirely disappeared. 

 We may transform each colour and its corresponding spectrum into the 

 other ah lihitmn.' 



' Lecoq de Tloisbaudran, Sjwctrcs Lumineux, Paris (Gautbier Villars). 



