WITH A FOURTEEN FEET REFLECTOR. 177 



18. Nebula h. 2008. 



1839. ^ug. 1.— Observed nebula M. 17. Nearly all the parts figured by Herschel in 

 the Phil. Trans, are distinctly and beautifully seen. The night has evidently improved. 



The two knots which Herschel describes in the appendix to his paper are well seen, but 

 the upper one not resolvable; the lower one is seen by Mr, Smith and myself oval, and ex- 

 tending downwards. I think there is a small branch at the angle of the nebula, in a down- 

 ward direction towards a coarse collection of large stars just below. A careful diagram made 

 of this and other particulars. 



^ug, 3. The curved part of this nebula is certainly smaller in proportion to the bright 



following branch than is represented in Herschel's figure. The extremely faint preceding 

 branch neither of us are able to see, although guided by Herschel's figure. 



^ug. 7. There is a coarse collection of stars below this nebula, in which I have once or 



twice suspected faint nebulosity. The branch before spoken of as extending downwards 

 from the smaller of the two knots, seems to pass to the right hand part of this cluster, and, 

 perhaps, running through it, to rise again, returning to the bright arm of the nebula, near the 

 star (29.) This is but a bare suspicion, for an assemblage of stars often gives a deceptive ap- 

 pearance of nebulosity among them.* 



" There is, I think, a faint ray from the internal angle of the nebula, towards the upper 

 star of Herschel's figure (25 of Plate VI.)" — S. This observation of Mr. Smith's it was 

 too late for me to verify. 



^i(g. 10. — Messier 17. Figured the stars in this nebula, amounting to upwards of thirty; 

 also the principal features of the nebula. The bright following branch is remarkably devoid 

 of large stars; it is, however, thick set with extremely small stars, just beyond the limits of 

 distinct visibility; the places of three of these, after long attention, I have succeeded in fixing, 

 and, with time and patience, could obtain more, but have little of either to spare; the fatigue 

 of the eye, moreover, is extreme. 



The observation made by Mr. Smith, on the 7th, of a faint ray towards the star (25) is, in 

 a great measure confirmed; I see certainly the upper boundary of such a ray, but think it 

 melts into, or joins with, the bright branch of the nebula below, being like a thin veil, or 

 gauze of light, drawn up from the bright nebula, and stretched from the star to the internal 



* It is scarcely to be supposed that this very common illusion is due to the sympathy of the neighbouring parts 

 of the retina, which is only adequate to account for the usual aberration, or irradiation, of stars. If I might 

 hazard a conjecture, I should, perhaps, attribute it to the same optical effect by which Sir J. Herschel (Mem. 

 Ast. Soc, Vol. II. , p. 490) is inclined to explain the apparent recession of the nebula Orionis from the stars of 

 the trapezium situate within it; contrast with their strong light blotting out the nebula in their vicinity. So the 

 diffuse starlight, which always redeems the sky from perfect blackness, may, perhaps, among many large stars, 

 be faintly revealed by contrast with the portions effaced in the immediate vicinity of the stars. This is, at best, 

 a very doubtful explanation, and principally thrown out for the purpose of calling attention to the phenomenon. 



VII. 2 U 



