IN LETTERS, AND ORALLY FOR YEARS AND YEARS, readers of this department have been saying, sometimes strenuously, that they doubted greatly that Ellison's claim to be the originator of the auto-collimation test for objective lenses could be justified. His claim appears in Chapter XI of his part of "Amateur Telescope Making," where he says he "believes he was the first to devise, and he certainly was the first to publish," a simple means for testing objective lenses-the familiar auto-collimation test.

Your scribe did not, however, pursue the question to a finish before Ellison died. A letter from a reader who questioned the claim was once sent to him and in his reply, now lost or misplaced he said he now claimed only independent discovery. He was asked whether the text of "A.T.M." should not therefore be altered (he was always most jealous of alterations to that text) but did not allude to this question when he next wrote, hence nothing was done at the time.

In 1928, when it was first decided to include his chapters on the objective lens in "A.T.M." (second edition), and he was asked if this was satisfactory to him, he sent us a full typescript of these chapters, with a few small alterations. But he did not, while so doing, alter his claim to the discovery and original publication of the auto-collimation test.

In December, 1936, he died.

Some time later, while browsing through old files of English Mechanics, a communication from Ellison was found (April 4, 1924, page 166) in which he had even then, in Britain, altered his claim to "independent discovery." Had he changed his mind in the meantime?

In September, 1937, after Ellison had died, W. H. Newman, Ditchling, Sussex, England, provided the governing data. While browsing through old files of Engineering (London) for 1888 he had come across an abstract of an article about the celebrated Clarks of Cambridge, Massachusetts, in which the auto-collimation test was described, and that abstract was from an article in Scientific American! No date was given, and for some reason Newman's letter found its way into your scribe's archives without the article being hunted up in Scientific American back files.

Recently, the Newman letter was encountered, a search was made and the article found in Scientific American Sept. 24, 1887. (Why, you ask, didn't your scribe remember it? Because, while he was then on the way, he was not yet born.) A part of that article is reproduced in Figure 1. Matter pertaining to tests begins in column 3 of that figure, which should be read before proceeding.

The following is the continuation of the text reproduced in Figure 1: "If the smallest irregularity exists, it appears as a spot or ring or other area on

the glass.

"In the other method, which is shown in the illustration, the source of light is a minute bead or convex surface of glass, carried in the center of a sighting tube, about six inches long. The light from a lamp is received on this and dispersed. All is arranged otherwise as before. The pencil of light from this source, representing almost a mathematical point, is received and transmitted by the lens as before, is reflected from the mirror, and again transmitted.

"In these methods the rays of light pass twice through the lens, so that a doubling of the effect due to a misshape is obtained. The Clark process, therefore, is of twice the delicacy of the older methods."

So this is where you end when you start tracing the auto-collimation test-at the Clarks, who obviously made it public as long ago as 1887, through Scientific American. It seems probable that Ellison himself never was certain about the matter. He lived far from any large library where he could have run the question down, and probably had to depend largely on oral advices and loose suppositions put forth by others.

The same article, written obviously by some visiting member of Scientific American's staff but anonymously, as was the editorial custom of the time, indicates that the 36" objective lens for Lick has just been finished by the Clarks and it contains a wood-cut (Figure 2) which is also reproduced because, while the name of the famous Clarks is everywhere mentioned in optical literature, the men themselves have become vague and shadowy-their pictures seldom seen. Alvan Clark, the father (central figure), had died only a month before, in August, 1887. Alvan G. and George B. Clark were the sons.

To an amateur telescope maker, an outline of the elder Alvan's career, from Scientific American, September 3, 1887, should be much to the point. Son of a farmer, he was artistic and for nine years worked as an engraver for calico printers, and then as a portrait painter for 11 more years. At this time, Alvan G. Clark, his son, was studying engineering at Andover and became absorbed in telescopes. The father began studying astronomy and mechanics, in order to instruct the son. Together they made a reflecting telescope. Encouraged by a Harvard professor, they next tried objective lenses and made out so well that they devoted themselves thereafter to making telescopes. Their reputation grew, reached England, and Dawes-the Dawes of "Dawes limit"-ordered a telescope in 1853. It did such fine work that many foreign orders for similar 5 1/4" objectives were soon received. By 1860 the Clarks had tackled an 18", then the largest objective in the world.

Larger and larger objectives were made in later years as the firm became more and more famous-leading up to the Lick and Yerkes; lenses.

The story of the Clarks is told in much greater detail in G. Edward Pendray's "Men, Mirrors, and Stars." George B. Clark died in 1891, and the other son, Alvan G. Clark, in 1897. The Clark tradition then passed on to the Lundins.

Figure 3 is from a wood cut in the Scientific American article referred to above, where it was published with the following caption: "Machine used in polishing the Lick objective."

This mention of Lick makes your scribe's face blush to a deep long-winter-underwear red, for in the September number he stated that the donor Yerkes was buried in a crypt under the pedestal of the 40" refractor at Yerkes. It turns out that it was Lick who was thus buried, and at Lick Observatory beneath the 36" refractor. There, darn it, goes a good story.

TYROS are urged in "A.T.M." to make their first mirror about f/8, this being a sort of compromise between a number of partly conflicting considerations, but when making a second mirror it is not inadvisable to depart from this, either as a venture or for a specific purpose. For planetary work many prefer something like f/10 or f/12, which gives a larger image. E. White, Box 1, Ymir, B.C., Canada, has just made an 8 l/2" f/12 (Figure 4) and says that "its performance is very good, both optically and physically. I designed it," he continues, "for observing lunar and planetary detail but it performs admirably on stars as well.

"The equatorial mounting (Figure 5) is motor driven by a train of gears made as per the instructions on hobbing ('A.T.M.A.' page 365) with a tap."

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