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VIII.— On the Advantages to be derived from the Use of Metallic Reflectors for Sex- 

 tants and other Reflecting Instruments; and on Methods of directly determinim 

 the Errors in Mirrors and Sun-Shades used in Reflecting Instruments. By 

 John Adie, Esq. 



(Read February 17. 1845.) 



It has frequently occurred to me that the difficulty experienced by instru- 

 ment-makers, in obtaining for sextants and other similar instruments, reflecting 

 mirrors perfectly parallel in their polished surfaces, and also the greater difficulty 

 of procuring glass perfectly homogeneous in its structure, might be overcome by 

 the use of metallic reflectors. 



It is well known, that, from the want of perfect parallelism in glass mirrors, 

 there arises an error in the reading of such instruments, inasmuch as the emer- 

 gent ray does not pass out of the glass at the same angle as the incident falls 

 upon it, and that from the want of homogeneousness in the substance, and the 

 unequal refractions caused by the veined structure of the glass. 



Whether this structure of the glass arises from the process of its manufac- 

 ture, or the want of proper admixture of the component parts, before being cast 

 into plates, I am not prepared to say ; but in aU the plate-glass I have tried, by 

 polishing it on the edges, this structure was observed ; so much so, that the plates, 

 on being seen through perpendicularly to the plane of their surface, shewed ob- 

 jects perfectly distinct ; while objects, when viewed through the glass at right 

 angles to this plane, were seen with difficulty, distorted and twisted in all direc- 

 tions. Of such glass, the mirrors of sextants, and other reflecting instruments, 

 are made ; and it is easy to conceive how very erroneous the angles may be, par- 

 ticularly when the incident ray falls on the mirror at a low angle, as it does 

 when lai'ge angles are observed, as in lunar distances and the like ; while the 

 indistinctness of the image observed under these circumstances, detracts much 

 from the utility of instruments fitted with such mirrors. As a practical illustra- 

 tion of the above, if we take a number of objects, and observe with a sextant the 

 angles between each, then observe the angle between the extremes, suppose this 

 120°, it wiU be found, in the great majority of cases, that the sum of the angles 

 observed does not agree with the observed angle of the ext]-emes, which should 

 be the case. 



These errors, and sources of en-or, are obviated when we make use of metallic 

 reflectors, having their surfaces polished perfectly flat ; a matter of no very difficult 

 attainment in practice. 



VOL. XVI. PAET I. Q 



