520 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
prevent the temperature of the speculum from progressively increasing 1 from friction, 
at length a point will necessarily be attained, sooner or later, according to the hard- 
ness of the pitch, when the pitch will yield so as accurately to fit the speculum, and 
the polish will then rapidly improve: but should the temperature rise further before 
the polish is complete, the pitch will have become too soft to work a very true sur- 
face, and the speculum, even though in every other respect perfect, will not define 
very sharply. It is very evident that, under such circumstances, a steady tempera- 
ture could not be maintained, except where the heat evolved by friction exactly 
balanced that dissipated by radiation and conduction, and that the result even on the 
limited scale of Mr. Mudge’s and Mr. Edwards’s operations must have been uncer- 
tain ; but in working large specula, the uncertainty was so great, that it gave rise to 
difficulties which I found it impossible to combat, and therefore I resorted to the 
simple expedient of making the speculum revolve in water, kept at an uniform tem- 
perature, generally 55°: all change also in the figure of the speculum, from variation 
of temperature' during the process, was thus at the same time prevented. The hard- 
ness of the resinous surface was therefore adjusted to suit the temperature, which is 
thus easily effected. Common resin is melted, and when nearly boiling, spirit of 
turpentine is added to it, perhaps about one-fifth of its weight ; but resin varies so 
much in quality, that there is no guide except actual trial. When the mixture has 
been incorporated by stirring, a cold piece of iron is to be immersed in it, and then 
placed for some minutes in a vessel of water at a temperature of 55°; if then a mode- 
rate pressure of the nail makes a decided impression without splintering, it is of a 
proper hardness for the first layer on the polisher, and only requires to be strained 
through canvas. I know of no mode which in practice answers better than the very 
rude one of judging of the hardness by the effects of the pressure of the thumb nail : 
there are others more precise, but they all take too much time, and sufficient accu- 
racy can be attained without them. For the second layer, it is mixed with one-fourth 
of wheat flour, which, by increasing its tenacity and diminishing its adhesiveness, pre- 
vents that accident complained of by practical men, viz. the separation of minute 
particles of pitch from the polisher, wdiich afterwards run loose between the polisher 
and the speculum. It is to be boiled till the water of the flour has been expelled, 
and the mixture becomes clear, and the boiling further continued till some of the 
turpentine has been driven off, and the mixture has become so hard, that at a tempe- 
rature of 55°, a very strong pressure of the nail makes but a slight impression : it is 
still too soft, and 1 then add to it an equal weight of resin; it will then be hard 
enough to produce a very true surface, and, at. the same time, soft enough to suffer 
the particles of polishing powder to imbed themselves, and consequently to produce 
a very fine black polish. Whenever the resinous mixture is remelted, I suspend the 
vessel to the beam of a scale, counterpoise it, and take care to apply the heat so gra- 
dually as not to drive off any of the turpentine, which is immediately perceptible by 
the disturbance of the equilibrium. To apply the resin, the polisher is first heated to 
