Intelligetice and Miscellaneous Articles. 229 



{the latter as stated by Mr. A. to have been determined by himself 

 in conjunction with M. Arago in Paris). 



The reflecting circle consists of two circles, of about six niches 

 diameter, revolving one within the other, the outer of which carries 

 the verniers, on which the divisions of the inner are read oft' to 20". 

 To each of these circles, in a plnne perpendicular to its own and 

 intersecting it in an exact diameter, is attached along narrow plane 

 reflector, in breadth equal to half the object-glass of the telescope, 

 and in length, nearly to a diameter of the circles, and so contrived 

 as to lie wholly below the plane of the limb, or at the backs of the 

 circles, and to" have their edges almost in contact, and intersecting 

 exactly in the prolongation of the common axis of both circles ; by 

 which means parallax is avoided, and the necessity of a reduction 

 to the centre, when the angles between near objects is taken, ob- 

 viated. The attachment of these reflectors to the circles, so as to 

 preserve the requisite freedom of motion and stability of the centre- 

 work, is performed by a frame-work easier to imagine than to de- 

 scribe without a figure. The telescope is attached to the limb of 

 the vernier circle \which is either held in the hand by a proper 

 handle, or supported on a pillar and counterbalanced). Its object- 

 glass is covered half by one of the reflectors and half by the other; 

 and the images of two objects seen by reflexion in each, being 

 brought to coincide, it is manifest that the angle between the re- 

 flectors (which is equal to half the angle subtended by the objects 

 at the centre of the instrument) will be read off" on the divided limb. 

 The principal advantages offered by this construction are, the avoid- 

 ing of parallax, and a power of measuring the same angle on four 

 different parts of the limb (independent of the four readings of 

 the verniers). 



The prismatic reflecting sextant is nearly on the same principle, 

 only that the internal reflexion at the bases of right-angled glass 

 prisms is here employed in the place of metallic or silvered glass 

 mirrors. It has, we Ijelieve, been described in the Correspondence 

 Astronomique of Baron Zach, for which reason it is unnecessary to 

 give any further account of it here. 



XL. Intelligence and Miscellaneous Articles. 



AMMONl.V IN NATIVE OXIDE OF IRON. 



THE formation of ammonia by the mutual action of iron, atmo- 

 spheric air and water, was long since shown by Austin. IVi.Che- 

 vallier exposed various specimens of native oxide of iron to heat, 



and taken |)art in, the determination of several of the powers of the 

 inicrosi-o[)e in (|uestion, under various coiribinations, in the presence of 

 .M. Aniici himself, ami several sjeiitlemen of the greatest experience in such 

 trials; when the results for four combinations tried were respectively ;500, 

 700, 1120, and .'$1.')!) (all for linear measure). With the last enormous 

 power, vision was still loln-ahli/ distiiui, though necessarily very obscure : 

 with all the olhiis, admirably peifect. 



and 



