The Story of a Missing Lens

by Dr. Ian Elliott

Many telescope lenses have had eventful histories but few can rivalthat of the 7.5-inch lens made in 1853 by Alvan Clark, the famousAmerican telescope builder.

Alvan Clark lived in Boston and was by profession an inventor andportrait painter. He had two sons - George and Alvan Graham - and twodaughters. His son George, while a student at Andover College,attempted to build a small reflecting telescope. This led to hisfather becoming interested in optics and eventually to the founding ofthe firm of Alvan Clark and Sons in 1850. Five times the firm madethe world's largest telescope with apertures ranging from 18.5 inchesfor Dearborn Observatory in 1862 to 40 inches for Yerkes Observatoryin 1897. The Yerkes instrument is still the largest refractor in theworld.

One of Clark's earliest customers was the Rev. William RutterDawes (1799-1868), the experienced English observer of double stars. After completing the 7.5-inch objective in 1853, Clark used it todiscover the faint companion of 95 Ceti. When Dawes learnt about this discovery he asked Clark to carry out further tests on other close binaries and then asked if he could buy the telescope. Clark reluctantly agreed and sold it to Dawes in March 1854 for $950. Dawes was very pleased with its quality and used it for two years, mostly for observing Saturn. Between 1855 and 1859 Dawes bought four more Clark telescopes and thus helped to establish Clark's reputation as a telescope maker on this side of the Atlantic from where it spread back to the United States.

In 1856 Dawes sold only the 7.5-inch lens to Frederick Brodie who remounted it at Eastbourne and later sold it to Charles L. Prince of Uckfield in Sussex. By 1869 the lens had been bought and remounted again by Dr Wentworth Erck of Sherrington House near Bray in Co. Wicklow. Erck made regular observations of sunspots from 1869 to 1888 and carried out more than 1900 measurements of 40 selected binaries between 1873 and 1880. In 1872 Charles E. Burton compared the optical performance of the 7.5-inch objective with a 9-inch mirror of silvered glass. At the opposition of Mars in 1877 Erck was one of the first in the British Isles to observe the moons of Mars which had been discovered by Asaph Hall only a few weeks previously with the great26-inch Clark refractor of the United States Naval Observatory in Washington. He also observed Jupiter and Saturn and, according to A. A. Rambaut, he was one of the first, if not the very first, to notice the proper motion of Jupiter's Red Spot. Burton, shortly before his death at the age of 35, used Erck's telescope to take photographs of the Moon.

There is ample evidence that the 7.5-inch Clark lens had a remarkable optical quality. The noted amateur, William Lassell, was "astonished" by its performance. Rambaut wrote about the lens as follows:
"It is in some respects a remarkable glass, being so full of bubbles that to one versed in such matters it might have appeared almost worthless for the delicate purpose for which it was intended; whereas it is remarkable for the exquisite definition it is capable of affording, as was proved on more than one occasion by the work done with it in the hands of Mr. Dawes and Dr. Erck."

According to Erck, Dawes considered the 7.5-inch telescope "moreperfect than any of its successors". After Erck's death in 1890, thetelescope was acquired by Sir Howard Grubb who sold it to William H.S. Monck for use from his residence at 16 Earlsfort Terrace inDublin. Monck was a lawyer by profession but had a keen interest inastronomy and wrote extensively in the current astronomical journals.In August 1892 Monck collaborated with George F. Fitzgerald and GeorgeM. Minchin in experiments to measure starlight photo-electrically.Minchin prepared the selenium detector, Fitzgerald probably suppliedthe quadrant electrometer and Monck provided his telescope. Badweather delayed tests but in the early hours of the morning of 28August 1892 Monck and his 26 year old neighbour, Stephen M. Dixonsucceeded in obtaining the first photo-electric measurements ofstarlight (1), thus anticipating by some 15 years the work of JoelStebbins in the United States.

In 1912 Monck presented his complete telescope to the Queen'sCollege Belfast (now Queen's University) but, despite enquiries, notrace of it has been found. In view of the historical interest ofthis Alvan Clark lens, it would be gratifying to obtain information onits whereabouts, if it still exists. If any reader has relevant information they are asked to kindly get in touch with me at Dunsink Observatory, Castleknock, Dublin 15, Ireland.

(1) I.Elliott, "The Monck Plaque" Orbit, September 1987 pp 6-9.

Vital statistics of the Alvan Clark lens:
Aperture : 7.5 inches
Focal length : 110 inches
Appearance : glass full of bubbles


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