large Reflecting Telescopes. 141 



With this alloy a speculum was cast fifteen inches diameter, 

 with a rim and ribs similar to the one before described, but 

 in every respect thinner, — not half its weight. It was turned 

 smooth and flat at one side and tinned. Six pieces of the 

 highest speculum metal were then prepared one quarter of an 

 inch thick, and fitted so as to make, when put together, a com- 

 plete circular disc fifteen inches diameter; these were then 

 arranged on the flat tinned surface of the brass speculum ; 

 the temperature was then very gradually and equally raised 

 till the tin was in fusion, and till every part of the under sides 

 and edges of the speculum metal was perfectly tinned. A slight 

 pressure was then uniformly applied, and the temperature gra- 

 dually reduced till the tin became solid. We then had a spe- 

 culum composed of zinc and copper plated with speculum 

 metal one quarter of an inch thick, adhering to it as firmly in 

 every part as if it had been one piece of metal. This metal 

 was ground and polished by the machine described in a for- 

 mer number of this Journal. It has a focal length of twelve 

 feet, and as there is a set off of about a quarter of an inch at 

 the edge, it has fourteen inches and a half clear aperture. It 

 far surpasses the other metal in the brilliancy and whiteness of 

 the image, as was of course to be expected. In other respects 

 it is the same, as they both bear distinctly a power of 600 

 at a printed paper, or at the cut stone pinnacles of a church 

 distant about 300 yards. There can be no mistake as to the 

 powers, as I make use of single lenses. Only one favourable 

 night has occurred since the stand and its appendages have 

 been brought to such a state as to render the instrument 

 tolerably manageable. The new moon was on that occasion 

 examined with powers from 80 to 600, and very perfectly 

 defined. The pole star, s Bootis and some other stars not re- 

 quiring high powers were well shown. From the defective state 

 of the machinery for giving motion to the telescope high 

 powers were not employed, and farther trials were deferred 

 till another opportunity. The stand is precisely similar to 

 Mr Ramage's, but the pulleys and some other appendages 

 are not complete. From a comparison of the instrument in 

 the day time with others which perform extremely well, I en- 

 tertain very sanguine expectations of its powers upon the dif- 



