204 Dr. and Mrs. Huggins. On the Principal 



burning magnesium, is placed. The lens at g is so placed as to bring 

 the light approximately to focus at the place of the slit. 



It is obvious that with this arrangement an extremely small shift 

 of the light before the hole h would be sufficient to cause the ray 

 reflected from the mirror to go off the slit, and that the reflected light 

 can pass into the slit only so long as its direction remains sensibly invari- 

 able relatively to the optical axis of the telescope. It is also obvious 

 that any flexure in the spectroscope, or in the tube connecting it to the 

 telescope, would affect similarly the light from the nebula and from 

 the magnesium. The precaution was taken, however, to so orient the 

 spectroscope, that any flexure from the weight of the instrument 

 would be in the direction of the length of the slit. 



The coincidence or otherwise of the direction of the light reflected 

 from the little mirror with the optical axis of the telescope can be 

 determined by comparing the spectrum of burning magnesium with & 

 in the spectrum of the Moon, or in that of the light of the sky. As an 

 additional safeguard in the comparison of the spectrum of the nebula 

 with magnesium, since my early observations had shown the nebular 

 line to be very slightly more refrangible, the mirror was purposely so 

 adjusted that, though the lines of the burning magnesium were seen 

 to fall upon the corresponding dark lines b in the Moon or sky, 

 yet a careful observation would show a very minute overlapping 

 of the bright lines towards the blue. This state of things would 

 diminish a little the interval which should be seen between the 

 nebular line, and the termination of the magnesium-flame band and 

 so make the observation more difficult. It is evident that if under 

 such circumstances of adjustment the nebular line were seen on the 

 more refrangible side of the magnesia band, the observation, being a 

 delicate one, would be more trustworthy, for in the case of coincidence 

 with magnesium the line would appear towards the opposite and less 

 refrangible side of the magnesia line, broadening the line towards the 

 red (loc. cit., p. 49). 



The stability of this adjustment depends upon the rigidity of 

 position of the little mirror within the telescope; as this weighs only 

 a small fraction of an ounce, and is supported by a strong steel arm 

 firmly attached by four screws to the steel tube of the telescope, there 

 is an almost complete absence of any chance of its displacement. 

 During twelve months not the smallest alteration has been detected, 

 though very careful examinations have been frequently made. 



At the time the comparisons were made last year, namely, in 

 March, the nebula was getting low, and from perhaps an excess of 

 caution I described them as follows : " Although I consider the 

 results to be satisfactory, I prefer to say that I and Mrs. Huggins, 

 independently, believed fully at the time that we saw the appearance 

 which all former observations of the line led me to expect, namely, 



