312 
MR. W. LASSELL ON POLISHING THE 
further augmented by the slight increase of temperature acquired by the pitch in the 
act of polishing. 
Acting on this principle, which has been supported by pretty extensive practice, I 
usually set the crank H, for polishing a twenty-four-inch speculum with a focus of 
ten diameters, to a radius of 3‘0 inches, and the crank E to a radius of 2 -75 inches, 
with a speed of motion of H producing about seven revolutions per minute. The 
pulley on the main shaft, in connexion with the largest speed-pulley I, of fig. 1, Plate 50, 
is about 4T inches diameter, causing the speculum to revolve once in about 3 m 45 s 
against the sun ; and the speed-pulleys on the long arm C of fig. 8 have their bands 
so arranged as to cause the polisher to revolve on its axis once with the sun in 
about 5 m 8 s , none of these bands being crossed. The band on the pulley on the 
vertical shaft H, connecting it with that on the vertical shaft E, is necessarily crossed 
to increase its friction. A slight addition to the counterpoising weight of the long 
arm C lifts its end about 2 inches above its ordinary working position, so that there 
is now room to place the polisher upon the speculum already charged with rouge. 
By restoring the counterpoise the pin may be introduced into its socket on the back 
of the polisher and the engine started. Pretty frequent supervision of the operator 
will now be necessary through the whole process, which may last three or four hours ; 
and perhaps the lustre may not be brought up to its maximum even in that time. 
I generally allow the machine to proceed uncontrolled until the powder begins to dis- 
appear from the intermittently exposed margin of the speculum. By this time the 
powder, as it works slightly over the edge of the speculum or adheres to it, ought, if it 
has been working well, to have become considerably darker in hue, so as to tend to 
a purplish colour, from the admixture of minute metallic particles. Nothing like 
dryness of the edge must be permitted to supervene. The engine should now be stopped 
and the polisher withdrawn by sliding it off the speculum. Probably the incipient 
polish may be somewhat more advanced towards the exterior of the speculum, which 
is what is desired ; but it should not be very strikingly so, and the advance should be 
regular, not sudden. It is not well to add powder while the machine is going ; it cannot 
then be added uniformly over the whole surface, and it tends therefore to irregularity 
of action. The aspect of the polisher should be examined. Some part at least of every 
square ought to have been obviously in contact, and an oblique view should reveal no 
apparent inequality of texture. As rapidly as convenient, and before the squares can 
dry, the powder which may have lodged in the grooves should be distributed evenly 
over the whole surface ; and after the speculum has had another pretty free dose of 
powder, the polisher should be placed upon the speculum and the process be carried on. 
When the powder has apparently been again used up and has nearly disappeared from 
the speculum, the polisher may again be removed. And now the lustre ought to be very 
considerably advanced, and the little powder adhering to the edge of the speculum be 
of at least a deep purple colour. The powder may be lightly and quickly removed 
from the grooves in the pitch with the camel 5 s-hair brush, and distributed over the 
