INVESTIGATING A TELESCOPE BY H. TULLEY Peter Abrahams, e-mail: telscope@europa.com This presentation will describe two aspects of an investigation into an antique telescope. The telescope itself was examined, using resources available to an amateur scientist with a null budget. Secondly, the maker was researched, using the very limited library resources of an Oregonian, and relying on remote scholarship via mail, phone, and e-mail. I acquired this tabletop terrestrial telescope, signed H. Tulley, in a trade that included a very rare surveyor's compass. Measurement revealed it to have a clear aperture of 2 5/8", and a focal length of 40 to 42", for an f ratio of 15. Magnification is about 40 power. The first test was of course observation, of the world and of test objects. The finest lines visible through a telescope give a rough idea of quality. The pattern can be moved to the edge of the field to test for astigmatism or field curvature, and all telescopes show some degree of aberration at the edge of the field. These patterns are also given in red, yellow, and blue to check color correction, although this test is more difficult to observe and interpret. Edmund Scientific, among other suppliers, sells these charts. The contrast of an image is equally important, and in inferior instruments, colors are pale and details are lost from dark objects by an overall haze. This is caused largely by scattered light from poorly polished objectives, although it can be reduced by proper baffles in the telescope tube. A quantitative test of contrast involves the fearsome Modulation Transfer Function. The untutored amateur can test contrast by viewing a shaded area from a brightly lit area, such as a forest from a field or a garage from the street. Antique optics will flunk this test unless scrupulously cleaned, and a new pair of binoculars will give a comparison. Finally, with a modicum of training, the star test is the single best test that can be applied to a telescope. All these procedures were performed in grave ceremony on the Tulley, and I am pleased to say that it passed the resolution test with colors flying, though I couldn't locate a Union Jack in far off Portland. Antique glass is not homogenous like the best modern optical glass. I used a 10 power loupe magnifier to closely examine the objective, and found two layers of very tiny air bubbles, one layer in a sheet several millimeters wide near the edge and one on the inside surface of one of the elements. I could also see a sheet of denser glass in a small area of one lens element. The artistry of the early telescope makers was in large part focused on polishing small corrections to the lens surface to allow irregular glass to focus on a sharp point. Profilometers and interferometers can be used to reveal the profile of the lens surface, and interferometers are within the reach of the ambitious amateur scientist. Autocollimation is the best technique for revealing the irregularities within the glass. This is a relatively simple test that requires only an optical flat and a Foucault tester. In keeping with the rustic simplicity of this presentation, neither test was performed on the Tulley. However, viewing the objective through crossed polarizers gives a view of the striae that can degrade performance. It also shows stresses in the glass from rapid cooling or mechanical pressure. These stress patterns sometimes do not degrade optical performance, but they can make it very difficult to figure the lens, and can make the objective brittle and so should be noted. This simple test can be performed with two polarizing camera filters or sunglass lenses, but a curved polarizer, like a lens from spectacles, will not extinguish light across its whole diameter. The two polarized lenses are rotated so no light gets through them, and the lens in question is placed between them. In this illustration, a pattern of stress is shown from one point on the edge, and a streak is seen running through the middle. A close-up gives a better view but cannot show the colors and beauty of the patterns. Mineralogists use these colors to deduce quantitative measurements from thin slices of rocks, and it is possible that there is information coded in the color patterns of stressed glass, but I know of no references in this area. The streak is caused by a line of glass of different density than the rest of the lens, from inadequate stirring of the molten glass. It would be fascinating to place the lens in an interferometer to see if the maker corrected the lens surface to compensate for this striae. The stress pattern caused me to take a close look at the edge of the objective at that point. I found a round notch ground into the edge, barely visible when the lens was in the cell until a strong light was applied. I do not know if Tulley ground it in an attempt to relieve the stress; however, the notch is in both lenses and fits around a piece of wire soldered to the rim of the cell, so that is doubtful. There is no evidence that Tulley tested for stress, although by 1870 the Clarks were using polarized light to test their glass. Light from the sky is partially polarized, and Clark reflected skylight off the backside of a lens, so that it travelled through the glass twice, and then viewed it with a Nichol prism. If references to polarized light can be found in British glass maker's documentation of circa 1840, I would have to think that Tulley could have stress tested this lens. However, since the notch is in both elements and is aligned with the wire on the cell, it is probably designed to insure that the two lens elements are oriented correctly in relation to each other, and not rotated on assembly. If this is true, it means that the elements were corrected to a very high degree, or at least that an attempt was made. One common gremlin infesting antique telescopes is the wrongly assembled objective, a victim of an earlier restoration or cleaning. Was Tulley anticipating the ubiquitious clumsy restorer of today? Finally, the color of the glass in this instrument is the classic deep green. This is often taken as an indication of age, and it is certain that instruments of the later 1800s used more colorless glass. This is probably from contamination from iron used in glass crucibles, although some writers feel that is was purposeful, in an attempt to optimize color correction by filtering out red and blue light. I am intrigued by the notion that a spectroscope might reveal the chemicals in the glass that cause the color, but that procedure violates the rustic proviso of this research, and intrudes on the lazy parameter as well. This photo of the objective shows the color nicely. Photographs are of great import to any research in this field, both to document tests and to send off to the authorities in attempts to garner information. Photographing an objective is quite tricky. The thickness of a lens can be shown because the edge is rough ground, as in my photo of the notch. Glass color can be made visible with a white background. To a very limited degree, the lens profile can be deduced from reflections of several light sources. This lens is the finder objective and is very slightly convex, another is from the erector assembly and has a much steeper profile. Higher power lenses reflect smaller images, and flat lens surfaces appear solid white. Engravings usually photograph very well, and the style of the letters, and the crudeness or perfection of their execution, are clues beyond the name itself. Quite a few instruments are mistakenly engraved or misspelled. Names were engraved on instruments made elsewhere but sold by the named instrument maker. For example, the Dollond family occasionally put their name on microscopes that were made by another maker but sold by the Dollonds. I do not know of any references on engraving styles, but I am sure that an experienced instrument historian can acquire a feel for the proper engraving style for an era or location. Internal construction of the instrument can be documented, and here is one of the baffles in the Tulley tube. Cases are not always original, but when they have period hardware, they can often be dated. There is a very large amount of reference material on antique furniture, and the dating of various styles of construction and hardware is quite established. Telescope collectors who are lucky enough to find an instrument with documentation in the case will certainly want to photograph the paper. This note is pasted to the lid of the case for the Tulley, and complicates the documentation considerably, since it indicates that the telescope was made by Charles Tulley, when the instrument engraving suggests Henry Tulley. It reads, "Made by old Tulley of Islington to order and by him considered (for its size) as one of the best telescopes in England." One couldn't ask for a better inscription than that, although there are no guarantees of authenticity in this venue. This leads us to the historical, library-style investigation into this telescope, but before we leave the physical evidence, we can use the photographic art to remind ourselves that there is nothing more attractive than an old telescope. The first book stop for telescope research is Henry King's The History of the Telescope. King notes that Charles Tulley's early work is unknown, but by the 1820s, he was a respected instrument maker in Islington, part of London. Sir James South entrusted him with the grinding of a 5 inch objective and considered the resulting lens to be the finest in the world. King mentions two sons, Thomas and William, but no Henry Tulley. Thomas Dick, in the Practical Astronomer, notes Mr. Tulley, but adds little substantial information. William Kitchener's several books provided no further evidence. However, there is currently much more literature available on microscopes and barometers than on telescopes. Henry Tulley made barometers, and his name and addresses appear in Nicholas Goodison's English Barometers. Several microscope objectives signed H. Tulley appear in the Microscope Catalog of the Whipple Museum of the History of Science in England. The Mathematical Practitioners of Hanoverian England, by E. G. R. Taylor, adds that Charles Tulley bought the workshop and tools of Benjamin Martin, and speculates that H. Tulley might be a grandson of Charles. Armed with these scanty facts, I wrote to the Whipple Museum, part of the University of Cambridge, and received a reply from well known instrument historian J. A. Bennett, noting that 'old Tulley' should have been Charles. Another letter, to the Science Museum in London, elicited a reply from C. N. Brown, curator in Classical Physics, also offering no new data but requesting some sample photographs of the telescope for their files. I am pleased to have contributed to the collection of this institution, and included a request for any future researchers to contact me. This mysterious situation held until last month, when after long delays I finally received my copy of a new reference, Directory of British Scientific Instrument Makers, 1550-1851, by Gloria Clifton but the product of 10 years of work by several researchers. More than 5,000 makers and retailers are listed, along with over 10,000 apprentices and associates. Using directories of trade associations and city directories, Clifton found much new evidence concerning the Tulley family. To the extent that these directories are accurate, it seems that Charles Tulley had two sons, William and Henry, and was related to Thomas Tulley. Charles Tulley's business lasted from 1799 to 1824, and was succeeded by Tulley & Sons in 1826, which in turn was replaced in 1830 by William and Thomas Tulley. Henry Tulley's working dates are 1822 to 1833, in Bath, about 60 miles from London. This newly released information allows us to speculate that Charles Tulley made the instrument to order, as attributed in the note pasted to the case, and that the final assembly was sold by his son. This is hardly the last word on this telescope, but many other antique instruments have a history as interesting, and their stories are waiting to be told. 5