On the Progress of Science As Exemplified in the Art of Weighing and Measuring: Being the Presidential Address Delivered Before the Washington Philosophical Society, December 10, 1887 - Softcover

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1888. Excerpt: ... Note.--The magnifying power of Sir Geo. Shuckburgh's microscope seems to be referred to a distance of twelve inches for distinct vision. The powers inclosed in parentheses are estimated upon the assumption that the respective micrometer screws had one hundred threads per inch. In the memoirs of the French Academy nothing is said respecting the method adopted by the Academicians for comparing their various toises; but in his astronomy, Lalande states that the comparisons were effected partly by beam compasses, and partly by superposing the toises upon each other and examining their ends, both by touch and with magnifying glasses; they being all end standards.1 For the definitive adjustment of the length of their meters, which were also end standards, the French Metric Commission used a lever comparator by Lenoir. In 1742 Graham used beam compasses, which he considered trustworthy to 0-00062 of an inch, in comparing standards of length; but at that time the French Academicians made their comparisons of toises only to one twentieth, or one thirtieth of a line, say 0-0030O of an inch, and it was not until 1758 that La Condamine declared they should be compared to 0-0l of a line, or 0-00089 of an English inch "if our senses aided by the most perfect instruments can attain to that."1 Half a century later, ten times that accuracy was attained by the lever comparator of Lenoir, which was regarded as trustworthy to 0-000077 of an inch.2 The heads of micrometer microscopes are usually divided into one hundred equal parts, and if we regard one of these parts as the least reading of a microscope, then in 1797, Sir George Shuckburgh's microscopes read to one ten thousandth of an inch; and the least reading of microscopes made since that date has varied from one twenty...