and forty-eight inches aperture, placed regions ahnost beyond 

 the scope of measurement within the reach of human intel- 

 lect. But as Short, in a spirit unworthy of his talents, took 

 care that his knowledge should die with himself, and Her- 

 schel published nothing of the means to which his success 

 was owing, the construction of a large reflector is still as 

 much as ever a perilous adventure, in which each individual 

 must grope his way. Accordingly, the London opticians 

 themselves do not like to attempt a mirror even of nine 

 inches diameter, and demand a price for it which shews the 

 uncertainty and difficulty of its execution. In Ireland we 

 are more fortunate, for a member of our Academy, Mr. 

 Grubb, finds no difficulty in making them of admirable 

 quality up to this size, or even fifteen inches ; but with all 

 his distinguished mechanical talent, he is believed to be 

 doubtful of the possibility of more than doubling this last 

 magnitude in perfect speculum metal. 



Under these circumstances, too much praise cannot be 

 given to Lord Oxmantown, who, in the midst of other pur- 

 suits, has found leisure for such researches ; and by a rare 

 combination of optical science, chemical knowlege, and prac- 

 tical mechanics, has given us the power of overcoming the 

 difficulties which arrested our predecessors, and of carrying 

 to an extent which even Herschel himself did not venture to 

 contemplate, the illuminating power of this telescope, along 

 with a sharpness of definition scarcely inferior to that of the 

 achromatic. 



The chief difficulties which are to be overcome in the 

 construction of reflectors, arise from the excessive brittleness 

 of the composition of which specula are made, and from the 

 necessity of giving them figures which shall be free from 

 aberration. The great mirror in the Newtonian form is 

 (if the eyepiece and plane mirror be correct) the conical 

 paraboloid. 



B 2 



