180 Mr. J. N. Lockyer. 



the nitrogen double, which according to Dr. Huggins is coincident 

 with the nebula line. As in his observations of the nebula Dr. 

 Huggins used eight prisms of 60, the Cooke spectroscope with eight 

 prisms and a telescope magnifying fifteen times was first employed- 

 An electric spark between magnesium electrodes was used, and the 

 length of spark was so adjusted that the nitrogen lines were visible 

 when a Leyden jar was connected with the coil and the magnesium 

 fluting when the jar was taken out of the circuit. The spark was 

 placed about 30 inches in front of the slit, and an image formed by 

 a lens of about 9 inches focus. In this way the chances of error in 

 measurement, due to changes in the direction of the light-source, 

 with respect to the slit, were reduced to a minimum. The spectrum 

 was faint, so that it was found necessary to have the slit rather 

 wide. Under these conditions the magnesium fluting fell on the less 

 refrangible member of the double green air line, as Dr. Huggins 

 observed the nebula line to do in 186S ; this was confirmed by my 

 assistants, and was seen by my colleagues Professors Thorpe and 

 Riicker. Twelve and sixteen prisms were subsequently used, and 

 with the wide slit, which it/ was then necessary to employ, the mag- 

 nesium fluting still fell on the less refrangible line of nitrogen. 



I would here suggest that in future comparisons of the spectra of 

 the nebulae with that of magnesium the quantity spark should be 

 employed for obtaining the fluting, as it is in no way fatiguing to the 

 eye. 



This comparison was repeated with a Steinheil spectroscope with 

 three prisms of 45 and a telescope magnifying sixteen times. In this 

 case there was less light lost than with the Cooke spectroscope, and the 

 observations were made with less difficulty. 



A small quantity of lead chloride was also introduced into the 

 spark, and the lead line was seen to be slightly more refrangible 

 than the edge of the magnesium fluting, so as to form a close double 

 with it. Using a small jar, it was found possible to obtain together 

 the spectrum of nitrogen, magnesium, and lead superposed, and under 

 these conditions the magnesium fluting was seen still apparently 

 coincident with the less refrangible nitrogen line, and the lead line 

 was a little more refrangible. 



The air spark so far employed was that obtained by using a small 

 jar ; the nitrogen lines were very fluffy and the spark was so feeble 

 that it was always necessary to use a wide slit. 



In subsequent experiments the jar spark between the two platinum, 

 poles inserted in a glass tube containing air at a slightly reduced 

 pressure was used. This gave the nitrogen lines very much thinner 

 than the ordinary spark in air, and when a larger jar was put in 

 circuit the spectrum was also brighter. A narrower slit could there- 

 fore be used and comparisons made with greater accuracy. 



