314 
MR. W. LASSELL ON POLISHING THE 
instrument answers the end perfectly, exposing the pure under surface of the pitch 
without sensibly diminishing its thickness. The polisher should be placed in an inclined 
position and plenty of cold water used. If not immediately required, the polisher may 
be kept for an almost indefinite time in a perfectly fit state for use, and amidst con- 
siderable changes of temperature, by keeping it suspended horizontally, and occasion- 
ally examining its surface to see if it requires to he kept face downward or the reverse. 
For examination of the figure of the speculum I have always chosen to remove it from 
the machine, place it in the tube and turn it on a bright star, which is doubtless a severe 
and even crucial test. And in addition to thus examining the image and penumbrse of 
a star with the full aperture, I am accustomed to expose successively different portions 
of the mirror to the stellar rays, noticing whether or not any different setting of focus 
is required. 
For this purpose I have a set of six diaphragms, exposing respectively a central disk 
and five concentric rings, all of equal area ; and I do not pronounce any mirror perfect 
in which the eye is not satisfied with the same focus for all these. I do not say that 
the same precision of image is to be expected from the extreme external annulus when 
all the rest of the mirror is blocked out ; but if the focus he set upon it, it ought to be 
the same as that required for the central portion. This I consider a severe test ; and 
beside revealing the character of the general curve, it affords a means of determining its 
regularity , by examination of the images formed by the intermediate annuli. A strict 
measure of any differences of foci which may be manifested will point out where the 
error lies. In the surface in question there may probably be a slight spherical error, 
which may be removed by another hour’s polishing with the same settings of the 
machine and a very moderate dose of the finest rouge. If, on the contrary, the 
figure should be hyperbolic, I should revert to the hone-tool, and repeat the process of 
polishing. 
From this description I think it will be seen that there is little that is irksome, 
tedious, or even uncertain in polishing by this process and with this machine a two- 
foot mirror. With a little experience and some mechanical aptitude, the operation is 
easy, and becomes very interesting ; for though to obtain the highest perfection of 
figure repeated trials may be necessary, the process is by no means disappointing — the 
figure, if reasonable care be taken, being always useful and to a considerable extent 
satisfactory. The principal part of the work can be done single-handed, though occa- 
sionally an assistant is required, chiefly in removing the speculum to and from the 
observatory. The speculum (weight about four hundred pounds) is lifted from the 
machine with a small pair of three-sheave blocks, placed upon a low carriage, taken to 
the observatory, hoisted with the same blocks on to a platform nearly level with the 
lower end of the tube when placed vertically, and finally lifted into its place by an 
elevating-screw placed centrally under the hack-plate. The time consumed by this 
operation, including fixing the levers of horizontal support and final adjustment, extends 
to about two hours. 
