196 



Profs. Liveing and Dewar. 



[May 12, 



observation, that the distance between successive images of the same 

 line is directly proportional to the dispersion and inversely as the 

 order of the spectrum. Our own observations of the positions of 

 the ghosts of the b group pf magnesium lines in spectra of different 

 orders agree closely with Mr. Peirce's theory, and two different 

 Rutherford gratings both give us the same results. The annexed dia- 

 gram (figs. 6 and 7) gives the relative positions of the first pair of 

 //hosts of each of the lines of the b group in the spectra of the third 

 and fourth orders, when the angle between collimator and telescope 

 is 45°. If this is compared with M. Ch. Fievez's map, it will be seen 

 that he has probably been deceived by these ghosts, both in the solar 

 spectrum and in that of the spark ; but as he does not state the angle 

 between his collimator and telescope, no exact comparison can be 

 made. These ghosts are sometimes very embarrassing when many 

 lines are in the field of view, but they may be detected by comparing 

 the spectra of different orders, as the ghosts have different relative 

 positions in the spectra of different orders. In the spectrum of the 

 third order the first ghost of b 2 on the more refrangible side falls on 

 6 4 , and that of & 4 on its less refrangible side falls on b 2 . 



The Magnesium-hydrogen Spectrum. 



In the " Proc. Roy. Soc," vol. 27, p. 494, and vol. 30, p. 93, we have- 

 recorded a series of experiments which led us to attribute to magnesium 

 together with hydrogen a peculiar spectrum. This spectrum we have 

 on no occasion been able to detect in the absence of hydrogen. Ob- 

 servations on the spark discharge in nitrogen, in carbonic oxide, and 

 in hydrogen, at reduced pressures, confirmed the results given in the 

 first-mentioned paper, when the discharge was taken in the gases at 

 atmospheric pressure. It was further shown that this peculiar spec- 

 trum could be reversed during the voltaic discharge in a lime crucible, 

 provided magnesium and hydrogen were both present, but not in the 

 absence of hydrogen. Likewise the flame of burning magnesium was 

 found to emit this spectrum when the combustion occurred in *an 

 atmosphere containing either free or combined hydrogen. In summing 

 up our results the following opinion was expressed : — 



" The experiments above described, with nitrogen and carbonic 

 oxide at reduced pressures, are almost if not quite conclusive against 

 the supposition that the line at 5210 is due merely to the lower tem- 

 perature of the spark in hydrogen. Prom De La Rue and Miiller's 

 observations it would appear that nitrogen at a pressure of 400millims. 

 should produce much the same effect on the spark as hydrogen at 

 760 millims. Now the pressures of the nitrogen and carbonic oxide 

 were reduced far below this without any trace of the line in question 

 being visible. Moreover, the magnesium line at 4481, which is not 



