686 EARL OE ROSSE ON THE CONSTRUCTION OF SPECULA OF 6-FEET APERTURE, 
The last speculum is but tons, and is therefore considerably weaker than its pre- 
decessor; and by carefully comparing the two specula at low altitudes, we have been 
made thoroughly sensible of the great importance of strength in preventing flexure. 
There are little irregularities in the action of the supporting levers, which are much 
more injurious to the definition of the weaker speculum than the other ; and although 
these irregularities may be susceptible of further diminution, I think there would still 
be sufficient gain to make it worth while to cast a third speculum considerably heavier 
than either of the others. 
In the ‘ Philosophical Transactions ’ for 1840 I have endeavoured to explain the prin- 
ciple upon which the bed of hoop-iron acts ; some, however, seem to have attributed 
larger effects to it than I have, and of a different kind. It has been supposed by some 
that a molecular change takes place, somewhat similar to that which has been observed 
in the case of very small portions of speculum-metal rapidly cooled, while by others the 
change has been compared to the “ chilling of cast iron,” to which I think it bears no 
analogy: cast iron when chilled becomes almost as hard as hardened steel; there is an 
exudation of graphite — in fact, a chemical change the exact nature of which seems to 
be imperfectly understood: there is no such change in speculum-metal, it becomes 
actually softer. To obtain sound castings, all which seems necessary is so to manage 
the process that solidification must begin at one surface and proceed regularly to the 
other. By employing the bed of hoop-iron the object is effected with certainty; but 
the engineer may employ other means, perhaps sufficient for the purpose, which, under 
varying local circumstances, may be cheaper and more convenient. 
Possibly some useful hints may be gathered from a slight glance at the successive 
steps by which w^e obtained a clear view of the principle by which the founder should 
be guided in making large castings of speculum-metal. 
About the year 1827, on commencing a series of experiments on speculum-metal, 
I procured a small flat speculum from Mr. Tully, and two similar specula from 
Mr. CuTiiBERT, as specimens of the art in its most advanced state. I also procured 
from Mr. Cuthbert several small unwrought castings of about two ounces weight to 
practice upon. Mr. Telly’s specula were cast in the ordinary way in sand, and polished 
with rouge : but Mr. Cuthbert’s were cast in contact with iron, and so cooled instanta- 
neously ; they were polished with putty. All the specula for Mr. Cuthbert’s micro- 
scopes were made in a similar manner. He was under the impression that speculum- 
metal cooled instantaneously was more suitable for his purpose than common speculum- 
metal — that it was sounder, more compact, and resisted better the action of emery. 
These specula were accidentally exposed to the air of the laboratory for a considerable 
time ; and at length we remarked that Mr. Cuthbert’s specula had somewhat lost their 
polish, while Telly’s speculum was as bright as ever. The inferiority of Cuthbert’s 
specula we attributed to an excess of copper, but with further experience we came to a 
different conclusion. We had several early samples of chilled speculum-metal, and 
corresponding samples of the same metal cooled gradually. They were obtained in this 
