1870.] 



of the Nebula of Orion, and of Jupiter. 



243 



Inaccuracy of focus of the image on the slit might perhaps somewhat 

 mislead, but this has not been allowed to come into play ; for the focus is 

 readily adjusted with considerable delicacy, by examination of the breadth 

 of star spectrum, which is reduced to a minimum. 



In Sir John Herschel's Cape drawing, a slight nebulosity is seen within 

 and immediately about the trapezium ; and in the description is found the 

 following extract from note-book : — " In the interior of the trapezium there 

 exists positively no nebulosity, at least none comparable in intensity to that 

 immediately without it." 



There being (as far as I can now see or remember) no other special refer- 

 ence to this matter in the description, it is not quite clear whether or not 

 the nebulosity in the drawing rests on this evidence or on that of other 

 nights when it may have been more conspicuous. 



In Lord Rosse's drawing the trapezium is enclosed in, and itself encloses, 

 a space totally free from nebulosity. My own telescopic observations here 

 (on not good nights unfortunately) indicate a positive though comparatively 

 faint nebulosity within and about the trapezium, somewhat as in the Cape 

 drawing ; the spectroscope, however, shows with much force that this 

 nebulosity not only exists, but is comparable in brightness to that surroun- 

 ding the trapezium at some distance, — the brightest part of the nebula 

 in fact ; and therefore that, in ordinary observation, the faintness or appa- 

 rent complete absence of nebula is mainly due to the disturbing brightness 

 of the four stars, and not to any intrinsic extreme faintness or absolute 

 vacuity. 



Jupiter has been examined with results, if not, as far as may be judged 

 at present, important at least interesting. Here, again, the large size of 

 image is brought into prominent play ; with the original Cassegrain image 

 the light is barely sufficient, but with the image condensed (at pleasure 

 within certain limits) fair work becomes possible, the spectrum being con- 

 siderably bright. 



The lines G, F, b, C, D, are seen without the slightest difficulty, C (being 

 near to visible limit) not so readily, but unmistakably, and many other 

 lines with attention. A marked feature is a dark nebulous band between 

 C, D ; from measures this turns out to be one of the bands examined by 

 Mr. Huggins, 882 of his scale* (C 6 of Brewster ?). 



The observations were made generally with Jupiter not far from the 

 meridian. On one night only of those employed was the atmosphere at 

 all free from perceptible haze ; as far, however, as memory could be trusted, 

 there did not appear to be any perceptible difference in the intensity of the 

 line on the different nights. This line is always so conspicuous that, were it 

 not for Mr. Huggins's more critical observation, I would be inclined to think 

 that in Jupiter it is much stronger than in an equally bright daylight spec- 

 trum, under conditions even more favourable than those afforded by the 

 * [Wrong identification : see next paper.] 



