122 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. Tn 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 difierential principle. On 

 the top of the axis is fixed a fiat disc or cup of beeswax which admits of 

 being easily turned true on the spindle itself, and surrounding this disc is a fiy 

 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 enabled 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 ofi* 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 

 capabiKties 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 such an angle of incidence that the 

 reflected rays shall descend vertically, such reflected rays will preserve their 

 parallelism, and the paraboloid will collect and reflect them upwards to the 



