120 Transactions. 
As the rays of light 
from the heavenly 
: cally taken as parallel, 
it is evident that this 
is the theoretical 
Fig. 1. š figure for the specula 
of reflecting Seana! aa for then only can all the rays of light be reflected to 
one point (F 
It will be chased that this curve, near its vertex, approaches to, although 
it cannot perfectly coincide with, the figure of the sphere. 
Now as this curve is a varying one it is clear that no grinding and 
polishing can mathematically produce the parabolic figure ; but as the curve 
in a sphere is invariable, therefore the spherical figure is first imparted to the 
ordinary speculum, and this is then modified empirically so that it shall 
approach to the parabolic figure. 
In this manner specula have been constructed whose diameters equal one- 
sixth of their focal length, but as the parabolic curve rapidly departs from the 
spherical it is evident that reflecting telescopes of large aperture on the 
ordinary construction must be of great length and cumbrous in their manage- 
ment. There is also a difficulty in giving them a perfect and durable polish, 
and then mounting them so that they shall neither be affected by changes of 
temperature nor deflection of different parts, from their great weight. 
The telescope here described has been constructed with a view of sur- 
mounting some of these difficulties ; its speculum may be said to be cast in 
Nature’s mould, as its figure is determined by the action of those “ Laws of 
Motion,” the truth of which were enunciated, and their universality demon- 
strated by Newton. 
Let any liquid be rotated in a vessel, with a given velocity, on an axis 
which has been adjusted perpendicular to the horizon. After a short time all 
the forces will be in equilibrium, and the fluid will assume a fixed position. 
As the surface is free to move, it must, at every point taken upon it, be 
perpendicular to the resultant of the forces acting upon it at that point. 
Let the curved line (Fig. 2) be a section of the 
rotating surface made by a plane passing through 
N V, the axis of rotation. 
Let P be any point taken on it. If P M bedrawn 
at a right angle to the vertical axis N V, it is 
evident that during the motion of the point P will 
is M. In consequence of this circular motion, a 
centrifugal force will be developed, pressing against 
