1893.] Bands in the present Spectrum of Nova Aurigce. 31 



not able to observe the spectrum of the Nova until the night of 

 February 1. 



Before this time the altered appearance of the spectrum of the 

 Nova had been observed at several observatories, and its spectrum 

 had been described as consisting mainly, in the visible region, of a 

 bright line in the orange, of the two nebular lines, and of the hydro- 

 gen line at F. 



As soon as we directed the spectroscope to the star, we saw at once, 

 even with one prism, that the two principal bright bands which had 

 been described as the " nebular lines " were, in strong contrast with 

 these, not single lines but broad bright spaces, diffused at the ends 

 and irregularly bright, which we suspected to be groups of bright 

 lines. 



On February 8 we observed these bright spaces with the 4-inch 

 grating of 14,438 lines to the inch, using the spectrum of the second 

 order. The collimator and the telescope have each an aperture of 

 2 inches, and the spectrum was viewed under a magnifying power of 

 23 diameters. Our suspicion was then confirmed, the bands being 

 clearly resolved into groups of bright lines upon a feebly luminous 

 background. 



On February 26, micrometric measures were begun of the positions 

 of the constituent lines of the groups, when unfortunately we dis- 

 covered that in consequence of flexure in one part of the instrument, 

 a shifting of the micrometer webs relatively to the lines of the 

 spectrum was liable to take place, and so make the measures uncer- 

 tain to about as much as 2 tenth-metres when the spectrum of the 

 second order was in use. 



The cause of the want of rigidity of the instrument in this respect 

 made it necessary that the spectroscope should go back to Messrs. 

 Troughton and Simms' workshops ; and then, from unavoidable 

 delays and the coming in of the Easter holidays, it was not until the 

 second week in April that the spectroscope was again in position for 

 use ; but by this time the Nova was too far past the meridian for 

 satisfactory observations to be made upon its spectrum. 



Our opportunities of working upon the spectrum of the star were 

 thus absolutely restricted to the few fine nights between February 1 

 and February 26 ; and, further, our observations of the positions of 

 the lines are, for the reason we have mentioned, affected with a 

 possibility of error which may be as great as 2 tenth-metres, though 

 it is probable that the positions given in the diagram are not actually 

 in error to as much as half that amount. 



For the same reason the resolution of the minor features of the 

 groups has not been worked out with the completeness which was 

 well within our instrumental means, if the number of fine nights had 

 not been so limited, for on some of the nights on which observations 



