62 MR ADIE ON THE USE OF METALLIC REFLECTORS FOR 
But, besides the avoidance of error, there are direct advantages in the use of 
such reflectors, which may be stated thus:—In the marine sextant, or reflecting 
circle, the reflection of faint objects is more easily obtained ; in other words, ob- 
jects are seen reflected by metallic mirrors which cannot be seen by the ordinary 
silvered ones. Another advantage is, that larger angles can be observed. This 
applies more particularly to the pocket or box-sextant, used in surveying, both at 
sea and land. From the small size of the index mirror, we cannot, when glass is 
used, reflect an angle much above 100°, the thickness of the glass cutting off the 
incident and emergent rays, when these fall on the silvered surface at low angles ; 
whereas, with the metallic reflector, the refiected angle can be obtained to its ut- 
most limit, or to about 140°, being nearly one-half greater than that which can be 
obtained by means of a silvered glass reflector. 
I am not aware of any account having appeared of the use of metallic mir- 
rors heretofore in the construction of such instruments, although I have little 
doubt, from the obvious advantages attending them, that the idea must have sug- 
gested itself to many others; and that the fact of their not having been brought 
into use, must be accounted for from the difficulty of obtaining speculum metal 
possessed of the requisite qualities. 
The liability of a highly-polished reflecting surface to be destroyed by tarnish 
and rust, from exposure to the atmosphere, and more particularly from exposure 
to the influence of sea air, is an objection that occurs on first view to the use of 
metal. 
All who are acquainted with the reflecting telescope, know how subject the 
mirrors of such an instrument are to deterioration from tarnish; and that, in 
many cases, even when due care has been taken of them, they have been alto- 
gether destroyed. Yet, it should be stated, that this is not the case with all such 
instruments. There are many reflecting telescopes, now very old, in which the 
mirrors are in a state of perfect preservation ; those I am best acquainted with, as 
having stood the test of time (and they are in general very good), having been 
made by the late James Short of London, who lived about eighty years ago. On 
the other hand, it is well known, that many speculum metals will not retain their 
lustre for many weeks under ordinary exposure. 
From these facts, we may infer, that it is the composition of the metal which 
causes the difference in the permanency of the polish. 
My attention was therefore directed to procuring pure metals to form the 
alloy or speculum metal. Tin is not difficult to be had in a state of great purity ; 
but it is otherwise with copper; for, as we advance in commerce, we find, that, 
day after day, this metal is brought to market more and more impure; so much so, 
that bar and cake copper of commerce are now so bad that they are nearly unfit 
for compounding as brass. 
The recently discovered process of electrotype, however, affords us the means 
