430 Dr. W. Huggins. On the Wave-length of the [Mar, 7, 



2000 c.c. It is possible, however, that nitrogen might be oxidised, 

 and thus not manifest itself under the above tests. In another experi- 

 ment the water of combustion was examined for acidity, but without 

 definite indications of nitric acid. The slight reddening observed 

 appeared to be rather that due to carbonic acid, some of which, it 

 must be remembered, would be dissolved in the water. These and 

 other matters demand further attention. 



The somewhat complicated glass blowing required for the combus- 

 tion apparatus has all been done at home by my assistant, Mr. 

 Gordon, on whom has also fallen most of the rather tedious" work 

 connected with the evacuation of globes and other apparatus, and with 

 the preparation of the gases. 



II. " On the Wave-length of the Principal Line in the Spectrum 

 of the Aurora." By WILLIAM HUGQINS, D.C.L., LL.D., 

 F.R.S. Received February 19, 1889. 



Notwithstanding the large number of determinations by different 

 observers, since Angstrom in 1867, of the wave-length of the prin- 

 cipal (and frequently the only) line in the spectrum of the Aurora, 

 this value has not yet been accepted as definitely fixed with the 

 degree of accuracy which is required for a final inquiry into its 

 chemical origin. The uncertainty within rather wide limits, which 

 seems still to obtain, has arisen mainly from the circumstance that in 

 nearly all cases the observations have been made with a small direct- 

 vision spectroscope, and under conditions which do not admit of an 

 accurate determination of the value sought for. About half the 

 number of some twenty-four observers agree pretty well, but among 

 the results given by the others the differences are very large in 

 relation to the accuracy which is required, though they are not 

 greater, perhaps, than was to be expected from the circumstances 

 under which the observations were made. 



I think it is very desirable, therefore, that I should put on record 

 some observations of the spectrum of the Aurora which I made in 

 the year 1874, but which up to the present time have remained 

 unpublished. These observations were made with a powerful spectro- 

 scope, and under conditions which enabled me to determine the 

 wave-length of the principal line within narrow limits of error. The 

 spectroscope was made by Sir Howard Grubb on the automatic 

 principle of his father, Mr. Thomas Grubb. It is furnished with two 

 "Grubb" compound prisms; each has 5 square inches of base, and 

 gives nearly twice the dispersion of a single prism of 60, namely, 

 about 9 6' from A to H. 



The object-glasses of the collimator and telescope are I 1 25 inch in 



