125, Transactions. 
at ordinary temperatures exists in the liquid state, and we possess in mercury, 
and possibly its amalgams, a surface of imperishable lustre ; and, when its 
equilibrium is established, then its perfection of surface may be safely taken to 
be such as no human skill could produce upon other metals, for no magnifying 
power, even that of the most powerful microscope, would be able to exhibit its 
surface by its irregularities, 
In telescopes of this description it is required : first, to construct a circular 
axis and concentric cup ; second, to fix it parallel to gravitation ; third, to 
give it an equable angular velocity. In the model before us will be seen the 
degree of approximation attained to these requirements. It consists of an 
upright steel axis about four inches long, the bottom of which rests on a fixed 
conical pivot, while the upper part (which has been ground circular) is kept in 
one position by a collar also ground circular. This collar admits of lateral 
adjustment by screws, which should work on the differential principle. On 
the top of the axis is fixed a flat disc or cup of beeswax which admits of 
being easily turned true on the spindle itself, and surrounding this disc is a fly 
wheel. In working this telescope it is first placed on a fixed base, and then 
levelled by placing a spirit level across the cup, turning the cup round and 
adjusting the screws till the bubble remains fixed. The axis is then truly 
perpendicular, and sufficient mercury is then poured into the cup and rotation 
communicated to it by any suitable power, in this instance a small electro- 
magnetic engine, the velocity of which is regulated by a conical pendulum. 
We are now eñabled to examine the printing placed on the ceiling of this 
room by magnifying its image, which is formed in the focus, by looking down 
into the mirror through the eye-piece ; although the mirror is rather small for 
this method of view, as the observer’s head cuts off those rays which descend 
nearest to the perpendicular, and which should consequently give the most 
distinct definition, the rays moreover are not strictly parallel as they would be 
if we were viewing a heavenly body, still we are enabled to judge of its 
capabilities by the definition it gives of these letters. When such an 
instrument is used for astronomical purposes the observations of course require 
to be made an object at, or within a few degrees from the zenith, these are 
always to be preferred for distinctness, on account of the rays traversing the 
shortest section of the atmosphere, the sweep of the telescope in Right 
Ascension being made by the earth’s rotation. 
It may be thought that we are debarred from obtaining a view of any part 
of the visible heavens at any given time by the use of a horizontal speculum, 
_ but such is not the case, for if the rays of light from any celestial body be first 
received on a large plane mirror at stich an angle of incidence that the 
reflected rays shall descend vertically, such reflected rays will preserve their 
- priiis and the paraboloid will collect and reflect them upwards to the 
