(52 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 reflected 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 fi-om 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 instniment 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 aU 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 weU 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 



