This important lens was purchased from Alvan Clark one of the most famous of the nineteenth century astronomical opticians. Made sometime between 1875 and 1885 the lens was used by H. C. Russell and Pollack in 1886 and 1887 to make experimental measurements of double stars using photography.
The perfection of the lens can be measured by the fact that in 1919 this lens was sent to Professor T. H. Laby, at the University of Melbourne, to aid his research on the work of fellow Australian Henry Joseph Grayson. Laby's primary focus was on precision experimental physics and the optical instruments essential to many scientific experiments. It was more than likely that the lens was used in connection with the plates made by the micro-ruling machine invented by Grayson in 1898. This machine was capable of ruling up to 40,000 lines an inch. Grayson's Gratings, as they became known, achieved international fame when the famous optician Carl Zeiss commissioned a plate with rulings up to 120,000 lines per inch.
References
Todd, David, P., Stars and Telescopes, Sampson Low, Marston, and Co., 1900
Knight, E., H., (ed), 'Knights American Mechanical Dictionary', Vol III, J.B. Ford and Company, New York, 1874
Evans, Joanne, Thomas Howell Laby, 1880-1946, cited http://www.asap.unimelb.edu.au/bsparcs/exhib/omp/people/laby.htm, June 2006
Bolton, H. C., Grayson, Henry Joseph (1856 - 1918), cited in http://www.adb.online.anu.edu.au/biogs/A090090b.htm, June 2006
Significance Statement by Geoff Barker, August, 2007
Alvan Clark (1804-1887) was one of the most famous telescope makers produced by the United States in the nineteenth century. Originally a portrait painter he built his first reflector in 1844 and two years later established a business with his sons Alvan Graham and George Bassett Clark. Together they made over 75 objectives which were over 6 inches in aperture each of which was considered to be nearly optically perfect and amongst the best produced at the time.
Among their many achievements were the building of an 18.5 inch telescope for Evanston, two 26 inch lenses for observatories in Washington and Virginia, a 30 inch lens for Pulkowa in Russia, a 36 inch lens for the Lick Observatory and the largest lens in the world at that time, a 40 inch for the Yerkes Observatory in Chicago. Making telescope lenses was an exacting and time consuming skill, the Washington lens for instance took over a year to grind and polish.
The time consuming process of used by Clark's when making their lenses is outlined in the following extract from Knight's 1874 American Mechanical Dictionary.
"The discs of Clark's lenses were made by Chance and Co. of Birmingham, England. The crucibles were made of clay, and built up gradually in rings of about 2 inches in height, the process requiring a whole year for its completion.
Optical glass of the best quality was then selected and crushed and the fragments separated according to their specific gravity by a hydraulic separator, in the manner employed for treating ores. Those of uniform quality and size are selected and melted by the most intense heat of a Siemens gas furnace; the mass is then cooled very slowly, and the central portion sawn out. This may be reheated until it is sufficiently fluid, and moulded to approximately the desired shape.
The discs are then tested, to ascertain if the glass is homogenous and free from flaws. This is affected by throwing the light from a lamp through a lens on one side of the disc, and placing the eye in the focus of the lens on the other side. Any imperfections thus appear greatly magnified, and if not removable by grinding, cause the rejection of the piece, at least for a lens of the size for which it was intended. The disc is ground upon concave plates of cast-iron of the proper curvature by pushing it back and forward at the same time giving a slow rotary movement. Emery, with water, is used as an abradant, finer sizes being successively used.
The polishing is effected by coating the tool with a thin layer of pitch, which is pressed into the proper shape; this is covered with rouge and water, and the disc manipulated as in the grinding process. The pieces forming the lens are, when finished, put together and set on edge, facing a luminous point placed at a distance equal to twelve to fifteen times the focal distance of the lens; the appearance of this point through the lens is examined with an eye-piece of high power, or by the eye placed in the focus; the optician thus judges what parts have an excess or deficiency of curvature. The polishing process is then repeated upon those portions as are too prominent."
References
Knight, E., H., (ed), 'Knights American Mechanical Dictionary', Vol III, J.B. Ford and Company, New York, 1874
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