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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 anda half inches it follows that it equals a New- 
tonian when their acting surfaces are as six toten: 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 
_bya 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 fulfilled. 
