518 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
The speculum was of course truly ground with the polisher first, and then the layer 
of pitch or resinous composition applied, the grooves remaining empty. 
There are two conditions which I have found essential in producing a successful 
result; the one, that the polisher should fit the speculum exactly during the whole 
process ; the other, that the resinous surface in contact with the speculum should be 
as hard as possible consistent with its admitting the polishing powder to imbed itself 
in it : without an attention to both of these, however accurately the motions of the 
machine may have been adjusted, however nearly a general parabolic figure may have 
been attained, the speculum will not define well. The first condition is satisfied by 
grooving the polisher, provided the resinous surface is sufficiently soft to expand late- 
rally into the grooves when necessary; but when it is so, I have not found it hard 
enough to give a very true surface, and therefore the second condition is not satisfied. 
But here it is necessary to explain the meaning which I attach to the words true 
surface , in contradistinction to accurate general figure. A true surface is one which 
observes the law of continuity, when, in fact, the normal to the surface everywhere 
cuts the axis in conformity to the law of the curve, whatever that may be. In prac- 
tice, the defect, which I call an untrue surface, is perceptible at a glance where it is 
very considerable, and the speculum is of long focus, for instance, twenty-seven feet; 
it is then only necessary to place the eye a few inches within the focus, while the spe- 
culum is turned to some bright object, as for instance the enamelled dial-plate of a 
watch, or the moon. The irregularity of the whole surface will then be apparent, 
more at the joints where the speculum is in separate pieces, but still more where 
there is a flaw or crack. The cause is obviously this : the metal under equal friction 
wears everywhere unequally, and therefore the inclination of the minute portions, I 
might almost say elements, of the surface, deviates slightly, but sensibly, and quite 
irregularly from the general curvature, producing an aberration independent either 
of general figure or aperture. A surface of speculum metal yields in the same irre- 
gular way to the action of acids, as indeed all metals do, but the more so as their 
texture is crystalline or fibrous. In proportion as the resinous surface is soft, and 
the polisher heavy, the irregularity increases, and therefore we should conclude that 
the harder the surface and lighter the polisher, the less the defect ; and such is the 
fact. The accuracy of the general figure depends mainly upon the motions of the 
machine and the thinness of the resinous surface. If the resinous surface is so hard 
that the particles of polishing powder no longer sink into it deep enough to be held 
fast, then the polish is destroyed, the polishing process passing into that of grinding ; 
long, however, before that limit of hardness has been attained, the resinous surface has 
lost its essential quality of expanding laterally, and therefore of preserving its exact 
coincidence of figure with the speculum. I have found that the two properties appa- 
rently inconsistent with each other, can be imparted to the polisher at the same time, 
simply by using the resinous composition of two different degrees of hardness, so as 
to form two very thin strata, the outer one being the harder. The resinous surface 
