Royal Astronomical Society. 5SS 



excellence when properly constructed — the Astronomer Royal re- 

 marked that his account would consist in some measure of a state- 

 ment of the differences between the processes of these two amateur 

 constructors. These differences, he thought, would be found well- 

 worthy the attention of all who were engaged in or contemplated 

 the construction of reflecting telescopes. It is certain that both 

 systems of methods are successful ; and it may be doubtful how far 

 the differences are connected with the difference of dimensions of the 

 telescopes : for in all that follows it must be borne in mind, that 

 Lord Rosse's largest telescopes are 6 feet in clear aperture and more 

 than 50 feet in focal length, while Mr. Lassell's are 2 feet in clear 

 aperture and about 20 feet in focal length ; that the thicknesses of 

 the specula are nearly in the same proportion as their diameters ; 

 and hence that the weights of the specula are nearly as 27 to 1 (that 

 of Lord Rosse's being about four tons, and that of Mr. Lassell's 

 being about three hundred- weight), a difference which in itself might 

 be expected to require some difference of construction. 



L The first difference is in the constitution of the metal-mixture 

 used for the speculum. 



In Lord Rosse's specula the metal is purely a mixture of tin and 

 copper, in a proportion understood to be an atomic proportion, the 

 weight of the copper being something more than double that of the 

 tin. In the process itself there appears to be strong evidence that 

 this proportion is truly atomic. When the metals are mixed in the 

 intended proportions, it is found that the addition of one or two 

 ounces of either metal to a mass of forty pounds of the mixture pro- 

 duces a difference so striking, that it is at once recognised by every 

 person who is employed on the works. The mixed metal, when in 

 fusion, possesses a remarkable union of penetrating power and of 

 viscidity. It is so penetrating that Lord Rosse found it impossible 

 to retain it in the ordinary cast-iron crucibles, which are cast with 

 the mouth downwards ; and he was compelled to have crucibles 

 made expressly for this purpose, cast with the mouth upwards. It 

 is at the same time so viscous, that when, in the casting of the spe- 

 culum, the liquid metal is poured upon what Lord Rosse calls a " bed 

 of hoops," that is, a broad base of the mould, formed by pressing 

 together the flat surfaces of a great number of iron hoops, whose 

 edges (trimmed into shape by turning and other mechanical opera- 

 tions) form the base, and through the interstices of which the air can 

 escape, the metal itself does not in the smallest degree enter into 

 these interstices. As soon as the cooling metal has acquired some 

 consistency, it is dragged into the annealing furnace, every part of 

 which has been brought to a low red heat. Great attention is given 

 to the form of the floor of this furnace, as the metal is still in such 

 a state that it receives figure from the furnace-floor. The furnace 

 is then closed (care being taken to prevent one part from cooling, 

 by radiation or convection, more rapidly than another), and after 

 some weeks it is opened and the metal is taken out. 



In Mr. Lassell's specula, the metal is made by first mixing copper 

 and tin in the same proportions as those employed by Lord Rosse, 



