200 Mr. J. N. Lockyer. On Photographs of 



consequence of the high wind, the slit was covered for an unknown 

 part of this time by the velvet used to keep out stray light, and this 

 was not at once discovered, as the finder for directing the telescope is 

 at the lower end of the reflector tube, away from the spectroscope. 

 This photograph only shows three or four of the more prominent 

 lines, but they are all sharply defined. The other photographs were 

 taken on February 2, 8, 9, and 10, the last with an exposure of three 

 hours. 



As a collimator has not yet been fitted to the tube of the reflector, 

 the exposure of the plate to the flame of burning magnesium was 

 made by closing the mirror cover, and burning magnesium at its 

 exact centre. One half of the slit was exposed to the nebula, and 

 the other half to the magnesium. 



Two prisms of 60 were employed. The part of the nebula photo- 

 graphed was the bright portion preceding the Trapezium. Tn some 

 cases, in consequence of clock irregularities, the stars of the Trapezium 

 have imprinted their spectra upon the plates, but these in no way 

 interfere with the spectrum of the nebula, since a longish slit was 

 used, and the spectra of the stars are narrow. 



There is a remarkable and almost absolute similarity between the 

 photographs obtained. The best one, taken on February 10, shows 

 all the lines of the other photographs in addition to others, and this 

 has therefore been selected for the determination of wave-lengths ; it 

 contains at least twenty-eight lines, about eight of them falling 

 between F and G. 



The principal lines are the three ordinarily seen in the visible 

 spectrum, the lines of hydrogen at G, ft, and H, and the strong line 

 in the ultra-violet near X 373. G is by far the strongest line in the 

 spectrum. The wave-length of the least refrangible line on the photo- 

 graph was taken as 5006 '5, as determined at Kensington, and this, 

 together with the hydrogen lines and the ultra-violet magnesium 

 triplet in the comparison spectrum, formed the basis of the curve for 

 determining the positions of the fainter lines. 



The lines next in importance to those already mentioned are near 

 wave-lengths 4470, 3890, and 3870. The first of these, the strongest 

 between F and G, is probably the line observed by Dr. Copeland, and, 

 as I have stated in a previous paper, is possibly Lorenzoni's / of the 

 chromosphere spectrum. There are also two fairly obvious lines 

 between F and 495. 



Amongst the fainter lines, the most prominent are near X 4027 and 

 4045, the former doubtless being the strong fine triplet seen in the 

 flame spectrum of manganese. 



Other still fainter lines are also shown, amongst which the most 

 interesting are the flame lines of calcium near 3933 (K) and 4226, 

 and lines near 4690 and 4735, which are probably the boundaries of 



