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glass a double refraction, most injurious to its performance in 

 an object glass. But supposing this and the equally probable 

 change of curvature from the weight of the lenses obviated, 

 still such an achromatic would be far below the six-feet in 

 quantity of light. From Amici's experiment with an object 

 glass of two and a half inches it follows that it equals a New- 

 tonian when their acting surfaces are as six to ten : this would 

 imply in the great one an aperture of fifty-six inches and a 

 focal length of eighty feet. But the absorption certainly in- 

 creases with the thickness of the medium, though neither the 

 law of this, nor the loss by the reflections at the four surfaces, 

 are accurately known. Mr. Potter found that a good object 

 glass by Dollond of four inches aperture and six feet focus 

 transmitted but 0.66 of the incident rays. This gives the ratio 

 of the equivalent surfaces 0.74, and it will be still greater where 

 the glass is three or four inches thick. It is said that the con- 

 struction of a reflector still larger than this is contemplated 

 by a northern Sovereign who has already shewn himself a 

 most munificent patron of Astronomy. If so, none will rejoice 

 more than Lord Rosse himself. It was not the mean desire 

 of possessing what no other possessed, or seeing what no other 

 had seen, that induced him to bestow so many precious years 

 on this pursuit : had such been his motives, he would have 

 kept to himself his methods, instead of opening his workshops 

 without reserve to all who had the slightest desire of following 

 his steps, and communicating in the most liberal manner the 

 fruits of long and painful experience. His sole object is to 

 extend the domain of astronomical knowledge : and the more 

 common such instruments become, the more perfectly will it 

 be fiilfilled. 



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