214 Dr. and Mrs. Huggins. bn the Photographic 



information a spectroscopic examination by photography of the nebula 

 in detail would probably give to us. 



These photographs, taken of almost the same part of the nebula as 

 the photograph of 1889, showed, to our surprise, the lines of hydrogen 

 at h and at H strongly impressed upon the plate, though these lines 

 were carefully searched for in vain in our former photographs ; in 

 them no trace of these lines could be detected, but the line near Gr was 

 strong, and there was present a large number of faint lines, of 

 about thirty of which the approximate measures were given in our 

 paper. 



The new photographs show not only the lines of hydrogen at h 

 and H, but also the first two lines of the ultra-violet series in the 

 white stars which I described in 1879.* Four of these lines had 

 been photographed in the spectrum of hydrogen by Dr. H. W. Yogel, 

 in 1879, and the entire series, with the exception of one, has been 

 since obtained by Cornu in exceptionally pure hydrogen. f 



The line * at X 3887'8 is strong, and the next line /8 at X3834'5, 

 though much fainter, is certainly present. There is evidence of light- 

 action on the plate at the position of the line 7 which we believe to 

 be present ; and we suspect, from traces of photographic action, that 

 one or more of the other lines of the white star series might have 

 come out with a longer exposure. 



It is not necessary to point out in the present note the importance 

 of the presence of these more refrangible lines of hydrogen in 

 respect of the view we have to take of the condition of things in the 

 nebula. In this connexion it is significant that the hydrogen lines 

 are sensibly stronger and broader on the plate as the Trapezium with 

 its stars is approached. 



Between the hydrogen lines and (3 there is a line stronger even 

 than a, which has a wave-length of about A, 3868. 



We do not find any line in the photograph exactly at the place of 

 the solar line K ; the position of this line appears to correspond to a 

 gap between two lines on the plate. We suspect the broad line on 

 the less refrangible side of the place of K would probably be resolved 

 by a narrower slit into two or more lines. 



The strong line which was first seen in our photograph of the 

 nebula taken in 1882 is certainly stronger than H<Y, and is by far the 

 most powerful line in the photographic region. On account of the 

 wide slit employed in my original photograph, I put the line at 

 about X 3730 ; from measures of the line in a photograph taken in 

 1889, with a narrower slit, we found that its position was more re- 

 frangible, and we gave the approximate wave-length " about X 3724." 

 There was necessarily some difficulty in determining its position 



* ' Phil. Trans.,' 1880, p. 669. 



f ' Journal de Physique,' 2nd ser., yol. 5, Aug., 1886. 



