Intelligence and Miscellaneous Articles. 311 



quantity, could be put in communication with the two poles of the 

 Dobbin. 



A small coil, giving a spark of a few millims. in free air, was 

 also used when a very feeble current was required. The experi- 

 ments were repeated a great number of times ; so that they present 

 a fair degree of certainty. 



Hydrogen Spectrum. 



Experiment 1. — The large coil was employed, with the condenser 

 arranged in tension, the dispersion of the spectroscope consisting 

 of six prisms, and the three lines C, F, and H being distinctly 

 visible under these conditions. The luminous pencil being then 

 progressively narrowed by means of a diaphragm pierced with an 

 aperture of 7 millims. diameter, on moving this diaphragm further 

 from the slit, and consequently bringing it nearer to the objective, 

 the length of the lines is observed to diminish, and at last the line 

 H becomes invisible, while C and F are still bright. 



Exp. 2. — Conditions the same ; dispersion of four prisms ; aper- 

 ture of the diaphragm 2 millims. By operating as before, the line 

 H is first extinguished, then the line C ; and on placing the dia- 

 phragm close to the projection-objective, the line F only is visible. 



Exp. 3. — Conditions the same, the same dispersion, condenser 

 arranged in quantity, aperture of the diaphragm 2 millims. The 

 same results are obtained ; but it is not necessary to move the dia- 

 phragm so far from the slit to cause C to disappear. 



Exp. 4. — With the small coil, without a condenser, and with 

 a dispersion of six prisms, the lines C, F, and H are well shown in 

 the spectrum. By then interposing the diaphragm of 2 millims. 

 aperture, the lines are observed at first to grow fainter and shorter ; 

 then the line H disappears, and afterwards the line C, the line F 

 remaining then alone visible. By employing a dispersion of six 

 prisms the same effect is produced more rapidly. 



Nitrogen Spectrum. 



It is known that the line spectrum of nitrogen, called also the 

 spectrum of the second order, is formed of several groups of lines, 

 indicated by Pliicker * by the numerals I., II., III., IY., and V. 



Exp. 1 . — With the large coil, the condenser in quantity, and a 

 dispersion of six prisms, group I. was faintly visible ; the others 

 were very marked. By interposing the diaphragm of 7 millims. 

 aperture, groups I., III., V., II., and the lines of group IV. with 

 the exception of the double line coinciding with that of the nebulae 

 37 HIV. and 73 H IV., are successively extinguished. 



Exp. 2. — Same conditions, dispersion of two prisms, and dia- 

 phragm of 2 millims. aperture. The same results were obtained ; 

 the groups successively disappear, and the double line alone remains 

 visible. 



If in any of the preceding experiments, at the instant that ex- 

 tinction of a line is produced the slit of the spectroscope be opened 

 wider, the line immediately reappears — which demonstrates that 

 the disappearance is caused by the weakening of the luminous in- 

 * " On the Spectra of Ignited Gases," Phil. Trans. 1865, plate ii. 



