1874.] of the Nebulae towards or from the Earth. 253 



therefore conveniently employed as a fiducial line in the observations I 

 had in view. 



In my own map of the spectrum of lead this line is not given. In 

 Thalen's map (1868) the line is represented by a short line to show that, 

 under the conditions of spark under which Thalen observed, this line was 

 emitted by those portions only of the vapour of lead which are close to 

 the electrodes. 



I find that by alterations of the character of the spark this line becomes 

 long, and reaches from electrode to electrode. As some of those conditions 

 (such as the absence of the Leyden jars, or the close approximation of 

 the electrodes when the Leyden jars are in circuit) are those in which the 

 lines of nitrogen of the air in which the spark is taken are faint or 

 absent, the circumstance of the line becoming bright and long or faint 

 and short, inversely as the line of nitrogen, suggested to me the possi- 

 bility that the line might be due not to the vapour of lead, but to some 

 combination of nitrogen under the presence of lead vapour. *As, how- 

 ever, this line is bright under similar conditions when the spark is taken 

 in a current of hydrogen, this supposition cannot be correct. 



A condition of the spark may be obtained in which the strongest lines 

 of the ordinary lead spectrum are scarcely visible, and the line under 

 consideration becomes the strongest in the spectrum, with the exception 

 of the bright line in the extreme violet. 



I need scarcely remark that the circumstance of making use of this 

 line for the purpose of a standard line of comparison is not to be taken 

 as affording any evidence in favour of the existence of lead in the 

 nebulae. 



Each nebula was observed on several nights, so that the whole observ- 

 ing time of the past year was devoted to this inquiry. In no instance 

 was any change of relative position of the nebular line and the lead line 

 detected. 



It follows that none of the nebulae observed shows a motion of trans- 

 lation so great as 25 miles per second, including the earth's motion at 

 the time. This motion must be considered in the results to be drawn 

 from the observations; for if the earth's motion be, say, 10 miles 

 per second from the nebula, then the nebula would not be receding 

 with a velocity greater than 15 miles per second ; but the nebula might be 

 approaching with velocity as great as 35 miles per second, because 10 

 miles of this velocity would be destroyed by the earth's motion in the 

 contrary direction. 



The observations seem to show that the gaseous nebulae as a class 

 have not proper motions so great as the bright stars. It may be 

 remarked that two other kinds of motion may exist in the nebulae, and, 

 if sufficiently rapid, may be detected by the spectroscope : 1. A motion 

 of rotation in the planetary nebulae, which might be discovered by placing 

 the slit of the instrument on opposite limbs of the nebulae. 2. A motion 



