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Updated: June 16, 2025
Madler represents them as similar in every respect, but Webb, observing them in 1855 and 1856 with a 3 7/10 achromatic, found them very distinctly different, Messier, the more westerly, being not only clearly smaller than its companion, but longer from W. to E. than from N. to S., as it undoubtedly is at the present time.
He sees no parallelism of condition, then, by which chance could act in forming a crystalline lens, which answers to the condition of an optician's shop, where it might be possible in many ages for chance to combine existing forms into an achromatic object-glass.
John Dollond, the son of a Spitalfields weaver, invented the achromatic lens in 1758, removing thereby the chief obstacle to the development of the powers of refracting telescopes; James Short, of Edinburgh, was without a rival in the construction of reflectors; the sectors, quadrants, and circles of Graham, Bird, Ramsden, and Cary were inimitable by Continental workmanship.
The only way to be sure of getting a good instrument is to try a number of them, but it may be well to know which are worth trying. Those made with achromatic glasses may be as much better as they are dearer, but we have not been able to satisfy ourselves of the fact.
Nevertheless, a 13-inch objective of the new variety was mounted at Königsberg in 1898; and discs of Jena crown and flint, 23 inches across, were purchased by Brashear at the Chicago Exhibition of 1893. An achromatic combination of three kinds of glass, devised by Mr. A. Taylor for Messrs. Cooke of York, has less serious drawbacks, but has not yet come into extensive use.
Before Sir Isaac Newton's optical discoveries, we could not tell why the structure of the eye was so complex, and why several lenses and humors were required to form a picture of objects upon the retina. Indeed, until Dolland's subsequent discovery of the achromatic effect of combining various glasses, and Mr.
With "achromatic glasses which magnified forty-five diameters" he examined the "pinholes" which covered the figure, and declared that "the beautiful finish of every pore or pinhole appeared to me strongly opposed to the idea that the statue was of modern workmanship." He also thought he saw the markings which Mr.
Frauenhofer, at the beginning of the nineteenth century, while applying Dollond's discovery to make large achromatic telescopes, studied the dispersion of light by a prism. Admitting the light of the sun through a narrow slit in a window-shutter, an inverted image of the slit can be thrown, by a lens of suitable focal length, on the wall opposite.
A 4 inch achromatic, or a silver-on-glass reflector of 6 or 6 1/2 inches aperture, will reveal on a good night many details which have not yet been recorded, and the possessor of instruments of this size will not be long in discovering that the moon, despite of what is often said, has not been so exhaustively surveyed that nothing remains for him to do.
Whenever we were at work with our microscopes at the Institute I always told her that her mind was the only achromatic one I ever looked into, I did n't say looked through. -But I did n't come to talk about that.
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