some of the Nebulce towards or from the Earth, 473 



I am not prepared to say that the coincidence is perfect ; on the 

 contrary, I believe that if greater prism-power could be brought 

 to bear upon the nebulae, the line in the lead spectrum would be 

 found to be in a small degree more refrangible than the line in 

 the nebulae. 



The spectroscope employed in these observations contains two 

 compound prisms, each giving a dispersion of 9° 6' from A to H. 

 A magnifying-power of 16 diameters was used. 



In the simultaneous observation of the two lines it was found 

 that if the lead line was made rather less bright than the nebular 

 line, the small excess of apparent breadth of this latter line, from 

 its greater brightness, appeared to overlap the lead line to a very 

 small amount on its less refrangible side, so that the more re- 

 frangible sides of the two lines appeared to be in a straight line 

 across the spectrum. This line could be 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 ob- 

 served, 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 possibility 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, however, 

 this line is bright under similar conditions when the spark is takeu 

 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 

 observing 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 nebula) observed shows a motion of 

 translation so great as 25 miles per second, including the earth's 

 motion at the time. This motion must be considered in the re- 

 sults to be drawn from the observations ; for if the earth's motion 



