51*2 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
There are here also some points which it is necessary to attend to. The perfect union 
of the plates with the brass speculum depends upon the fact, that if a plate of specu- 
lum metal, scraped as directed, is laid upon a clean surface of tinned brass, and the 
temperature raised a little beyond the fusing- point of tin, and then melted resin and 
tin applied, the plate of speculum metal will be immediately tinned all over, and 
the union of course will be perfect. If resin, however, is applied at the beginning- of 
the process, and therefore exposed for hours to an increasing temperature, the resin, 
before the temperature has reached the proper degree, will have been decomposed, and 
will effectually prevent the success of the operation. In my earliest experiments, I 
was not aware of this fact, and was therefore obliged to turn the plates over repeatedly 
with a wooden tongs, to remove the decomposed resin ; and, although where the plates 
were small this was practicable, in attempting to manage the plates of the three-feet 
speculum I failed : several were broken by unavoidable exposure to variation of 
temperature. I may perhaps as well mention, that formerly I tried the muriate of 
ammonia instead of resin, and also a variety of other processes, but none was com- 
pletely successful but the one I have given. The resin should not only be in fusion, 
but where the plates are large it is more prudent to regulate the temperature by a 
thermometer. The same observation applies to the tin ; I have found, however, that 
a portion of unmelted tin in the ladle, in contact with the fluid metal, was a sufficient 
guarantee against a too great disparity of temperature. The brick work should be 
perfectly dry, as a drop of condensed steam falling upon a plate would certainly 
crack it. 
Before I proceed to the grinding and polishing of the speculum, I will conclude 
the remaining experiments on the process of casting. The ease and certainty with 
which perfect plates of speculum metal of moderate dimensions had been obtained, 
obviously suggested a trial of the same principles on a larger scale. A perfect disc 
of fine speculum metal, twenty inches diameter, was immediately obtained, and re- 
cently so large a disc as three feet was cast perfect the first attempt, and has been 
finished without accident. 
Although the principles which guided the manipulation have been fully explained, 
there are some practical details necessary to success, which might not perhaps suggest 
themselves immediately to others who have not had the same experience in managing 
this intractable alloy that I have had ; a few remarks may therefore be useful. The 
disc, when cast, was about three inches and three quarters thick, and weighed about 
thirteen hundred weight ; the metal for it was fused in two cast-iron crucibles. In 
my earliest experiments, in consequence of the failure of a very large crucible of cast 
iron, one inch and three quarters thick, made for me at one of the principal London 
foundries, I concluded too hastily that cast iron would not answer. The first time 
the crucible was tried, after it had been about four hours in the fire, the speculum 
metal oozed through it ; when cold the defective portion was bored out and stopped 
with a screw ; the next time the metal oozed out in several places. The crucible was 
