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From thence their reflected rays meet with a little mirror, which sends them back to the ocular in such a way as to magnify the image produced. Thus in field-glasses refraction plays the principal part, and reflection does in the telescope. Hence the name of refractors given to the former, and reflectors given to the latter.
The result was a Newtonian of exquisite definition, with an aperture of two, and a focal length of twenty feet, provided by a novel artifice with the equatoreal mounting, previously regarded as available only for refractors. This beautiful instrument afforded to its maker, October 10, 1846, a cursory view of a Neptunian attendant.
The building was last remodeled and enlarged in 1911 when a reflecting telescope, with a 37-5/8 inch parabolic mirror, largely made in the shops of the University, was installed. In light gathering power this instrument is in a class with the Lick and Yerkes refractors, and it is at least as effective in astronomical photography, the purpose for which it was designed.
The present lighthouse is 30 feet higher than Smeaton's, and is fitted with the modern system of dioptric refractors, giving a light of 519,000 candle-power, which is greater than any other on the east coast of England. The need for a second structure has been obviated by placing the low lights half-way down the existing tower.
The perfect achromatism of a reflector is, of course, a great advantage, but the chromatic aberration of refractors is now so well corrected that their inferiority in that respect may be disregarded.
And since photographic refractors are corrected for the blue rays, exposures with them of orthochromatic surfaces would be entirely futile.
And even after Chester More Hall in 1729, and John Dollond in 1757, had shown that chromatic aberration could be nearly eliminated by the combination of a flint-glass lens with one of crown glass, William Herschel, who began his observations in 1774, devoted his skill entirely to the making of reflectors, seeing no prospect of much advance in the power of refractors.
The fortuitous combination of circumstances already described gave the matter a unique news value. Giant reflectors and refractors had been given and were in use in other parts of the world, but none so large or so important as this. The gift was sufficient to set Cowperwood forth in the light of a public benefactor and patron of science.
While the reflecting telescope was astonishing the world by its rapid development in the hands of Herschel, its unpretending rival was slowly making its way towards the position which the future had in store for it. The great obstacle which long stood in the way of the improvement of refractors was the defect known as "chromatic aberration."
Nevertheless, the exorbitantly long tubeless refractors, introduced by Huygens, maintained their reputation until Hadley exhibited to the Royal Society, January 12, 1721, a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpassed in manageability, one of the "aerial" kind of 123 feet.
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