196 Mr. J. N. Lockyer. 



was lower, and would become more like a line as the temperature was 

 increased. 



But this is not all ; a greater number of collisions per unit volume 

 at the same temperature would increase the visibility of the effects, 

 and greater brightness in a nebula may proceed from this cause as 

 well as from a less distance. We should not, therefore, expect to see 

 the fluting, even if its existence be conceded, in all cases, and the 

 smaller the dispersion the better it will be seen, cceteris paribus. Ex- 

 periments have been made here on the spectrum of magnesium when 

 seen very faintly with moderate dispersion. 



The conditions being such that the structure of the fluting near 500 

 was well visible when magnesium ribbon was burned in front of the 

 slit, a sufficient thickness of neutral tint glass was introduced to 

 reduce the brightness of the fluting until it was about equal to that 

 of the chief line seen in the spectrum of the nebula in Orion 

 Under these conditions, the 500 fluting is only faintly visible and the 

 secondary maxima entirely disappear. "We get only the brightest, 

 least refrangible member of the compound fluting, together with a 

 simple fringe of light without structure on the more refrangible side. 

 This experiment was shown at the Royal Society Conversazione in 

 May, 1889, and a note upon it may be found on page 13 of the pro- 

 gramme. The experiment has recently been repeated and fully 

 confirmed with a four-prism Sfceinheil spectroscope. It was found 

 best to adjust the dark glasses so that two or three of the maxima were 

 seen when the magnesium was burning ; then, when the magnesium 

 was just dying out, only the least refrangible one, with a slight fringe, 

 was seen. 



The greater luminosity of the first maximum and its fringe has also 

 been observed in another way. Magnesia, volatilised in the oxy- 

 hydrogen flame, with the proportion of gases properly adjusted, gives 

 the compound fluting pretty bright. If, then, the quantity of 

 hydrogen be increased or diminished gradually, whilst the oxygen 

 remains constant, the fluting gradually disappears, but the first 

 maximum and its fringe are seen when all the others have dis- 

 appeared. 



Numerous photographs have also been obtained which show the 

 first maximum brighter than any of the secondary ones. 



These experiments not only show that the first maximum is brighter 

 than the secondary ones, but further, that some of the fringe on the 

 more refrangible side of it is also brighter. In observations of nebulae, 

 therefore, if the chief line be due to magnesium only a very slight 

 fringe would be observed unless the luminosity be sufficient to render 

 visible some of the secondary maxima. 



I have shown, therefore, that many records exist as to the fluted 

 appearance of the chief nebula line records that amply justify the 



