242 Mr. A. Le Sueur's Spectroscopic Observations [Mar. 3, 



We may reduce these to a common epoch by applying to the former dip 

 the correction — 0'*96, this being the proportional secular change (as shown 

 by these six years) necessary to reduce the former epoch to the latter. 

 The former dip will therefore become 68° 6'«83 — 0 r *96 = 68° 5'*87. 



Reducing in the same way the horizontal force, we have 

 3*8346 + 0*00275 = 3*83735. 



The values thus become as follows : 

 From the April to September observations"! Hot. force. Dip. 



(reduced to epoch July 1, 1866) / 3*8373o 68 5 -87 



And from the October to March observa- j 3-83"20 68° 6'*41 



tions (corresponding to the same epoch) J 



The total force derived from the first series will therefore be 10*28717, 

 and that derived from the second series 10*29080, showing thus a difference 

 of 0*00363 in British units as the measure of the greater intensity of the 

 terrestrial magnetic force in the October to March period, than in the 

 April to September period. This is in the same direction, and very nearly 

 of the same amount, as that determined by Sir E. Sabine from the first 

 six years, which exhibited a similar difference of 0*00317 in British units. 



Thus we find that the two series agree in showing nearly the same semi- 

 annual variation for the total force, while the first period exhibits the 

 greatest semiannual variation of the dip. It ought, however, to be borne 

 in mind that the two series bear a different relation to the disturbance 

 period, the maximum of disturbances occurring about the middle of the 

 first series, and the minimum near the middle of the second. 



II. u Spectroscopic Observations of the Nebula of Orion, and of 

 Jupiter, made with the Great Melbourne Telescope/'' By A. Le 

 Sueur. Communicated by the Rev. T. R. Robinson, D.D. 

 Received January 27, 1870. 



In one particular the spectroscopic observations of the nebula of Orion 

 are not void of interest ; they show distinctly that considerable nebulosity 

 exists within and about the trapezium. The image at the slit is sufficiently 

 large to well separate the stars of the trapezium, so that when two of these 

 are, as it were, threaded on the slit, a clear space lies between them ; this 

 in the spectroscope gives the well-known lines with little, if at all, less 

 brilliancy than the general bright nebula. 



The small comparison-mirror being removed, the available slit is *4 inch 

 high, equivalent in the case of the Cassegrain image to about 43" arc; 

 with an image condensed about three times (which is the usual arrangement 

 and still allows sufficient separation), the slit may, therefore, be made to 

 considerably overlap the trapezium contour, and thereby, at the same time 

 as the trapezium, light from the brightest part of the nebula is under in- 

 spection ; it is curious to see that the spectral lines run with almost con- 

 tinuous brightness throughout the height of the slit. 



