+----------+------------+----------------------------------------------+ | Screw. |Wave-length.| Remarks. | +----------+------------+----------------------------------------------+ | 3·97 | 6562 |Moderately bright line. | | (5·88)| 5892 |Bright double line (Na). | | (6·25)| 5789 |Bright line (H). | |From 7·03 | 5591 |}Broad dull band of light; near 7·03 a | |to 7·55 | 5470 |} somewhat brighter line. | | 7·59 | 5461 |Bright line (H). | | 8·72 | 5231 |Dull stripe. | | 8·96 | 5187 |Broad misty stripe.} On a dull steady ground. | | 10·07 | 5002 |Faint line. } | | 11·05 | 4859 |Very bright line. | | 12·21 | 4709 |Moderately bright line.} On dimly lighted | | 12·75 | 4644 |{Line fainter than the } ground, becoming | | | |{ preceding. } fainter towards | | 13·28 | 5585 |Very faint line (H). } the violet. | | (15·71)| 4358 |Very bright line (H). | | 15·90 | 4341 | ” ” | +----------+------------+----------------------------------------------+ Here follow several more lines. In the first observations, the electric spark, about 1 centim. in length, was allowed to pass between platinum points in ordinary air. The sodium-line near 5·88 appeared continually. The bright double line at 10·03 and 10·07, with a weaker current or longer spark, was no longer to be recognized as a double line, but appeared as a broad somewhat confused line, of which the brightest part was near 10·05. No lines belonging to the platinum spectrum appeared. Ordinary rarefied air, under a pressure of 25 to 30 millims., and which was enclosed by mercury in a tube 8 millims. wide, showed exactly the same lines as Plücker’s nitrogen-tube (_b_), except that some lines belonging to the spectrum of mercury also appeared. This observation may be regarded as a confirmation of the conjecture above expressed as to the condition of Plücker’s tube III. (nitrogen). In the observations described under _b_, the air saturated with aqueous vapour was under a pressure of 22 millims. Besides the sodium-lines, lines of the mercury-spectrum appeared at 6·25, 7·59, and 15·71. The spectrum of rarefied air under similar pressure was found to accord completely with the spectrum of the light in the broad part of Plücker’s tube. III. (Nitrogen _b._)—A comparison of the spectrum of rarefied air saturated with aqueous vapour with the former shows the striking alterations in the spectrum which are brought about by the presence of the aqueous vapour.