26 STOKES. — SPECTROSCOPIC OBSERVATIONS. 



(2.) Spectrum of the Aurora. 



The spectrum of the aurora contains a well-known conspicuous 

 bright line in the yellowish-green, which has been accurately 

 observed. There are also other bright lines of greater refrangi- 

 bility, the determination of the positions of which is more difficult 

 accoimt of their faintness, and there are also one or more lines in 

 the red in red auroras. 



Advantage should be taken of an unusually bright display to 

 determine the position of the fainter lines. That of the brightest 

 line, though well known, should be measured at the same time to 

 control the observations. The character of the lines {i.e., whether 

 they are strictly lines, showing images of the apparent breadth 

 of the slit, or narrow bands, sharply defined or shaded off) should 

 also be stated. 



Sometimes a faint gleam of light is seen at night in the sky, 

 the origin of which (supposed from the presence of clouds) is 

 doubtful. A spectroscope of the roughest description may in 

 such cases be usefully employed to determine whether the light 

 is auroral or not, as in the former case, the auroral origin is 

 detected by the chief bright line. The observer may thus be led 

 to be on the look out for a display which otherwise might have 

 been missed. 



It has been said, however, that the auroral light does not in all 

 cases exhibit bright lines, but sometimes, at least in the eastern 

 and western arch of the aurora, shows a continuous spectrum. 

 This statement should be confronted with observation, special 

 care being taken that the auroral light be not confounded with 

 light which, though seen in the same direction, is of a different 

 origin, such, for example, as light from a bank of haze illuminated 

 by the moon. 



Sir Edward Sabine once observed an auroral arch to one side 

 (say north) of the ship, which was in darkness. Presently the 

 arch could no longer be seen, but there was a general diffuse 

 light so that a man at the mast head could be seen. Later still 

 the ship was again in darkness, and an auroral arch was seen to 

 the south. 



Should anything of the kind be observed, the whole of the 

 circumstances ought to be carefully noted, and the spectroscope 

 applied to the diffuse light. 



2. Polarisation of Light. By W. Spottiswoode, M.A., 

 LL.D., Treas. R.S. 



The fact of atmospheric polarisation, and the laws which regulate 

 it, are already well known. And it is therefore not probable that 

 observations upon it, although made under somewhat unusual 

 circumstances, will add materially to our knowledge. At the same 

 time, as the instruments are extremely portable, and the observa- 



