(reo a) 
VIII.—On the Advantages to be derived from the Use of Metallic Reflectors for Sew- 
tants and other Reflecting Instruments ; and on Methods of directly determaning 
the Errors in Mirrors and Sun-Shades used in Reflecting Instruments. By 
Joun 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 all 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 large 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 will be found, in the great majority of cases, that the sum of the angles 
observed does not agree with the observed angle of the extremes, which should 
be the case. 
These errors, and sources of error, are obviated when we make use of metallic 
reflectors, having their surfaces polished perfectly fiat ; a matter of no very difficult 
attainment in practice. 
VOL. XVI. PART I. Q 
