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    Re: Basics of computing sunrise/sunset
    From: Douglas Denny
    Date: 2009 Jun 18, 11:22 -0700

    Perhaps a longer account from the RS Crookes report might be worthwhile for interest's sake:-
    Douglas Denny.
    Chichester. England.
    The work of Crookes's group
    Given the uncomfortably hot conditions in which glassblowers worked, it was a 
    reasonable assumption that any damage done to their eyes was due to infrared 
    radiation. Burch began work immediately on an empirical study of whether 
    protective glass screens would allow workers to see what they were doing 
    while shielding them from damaging glare. Aided by his demonstrator, T. G. 
    Malpas, at the Physiological Laboratory in Oxford, Burch used Nernst 
    filaments as sources because of their high temperatures and steadiness of 
    light. These lamps were bought with the Committee's grant. The two men 
    gathered much useful data on the percentages of light transmitted through 
    various tinted glasses and also showed that transmission of radiation was 
    dampened when thin coats of metallic film were applied to glasses. 
    Unfortunately, such films caused blurring of images, which would have been 
    dangerous for any bottle blower in factory conditions. The application of 
    gauze screens in front of blue glass screens was also effective, but again 
    raised doubts as to whether optical definition would be compromised.14
    Meanwhile, Crookes undertook long-term research into how the addition of metal 
    oxides to glass recipes might produce a glass that would reflect infrared 
    light produced in the glare of white-hot furnaces. Made into safety 
    spectacles, such a glass would prevent, or at least reduce, damage to the 
    eyes of workmen fashioning molten glass. Crookes's investigations were partly 
    made at his home laboratory in Notting Hill with his assistant James Gardiner 
    (who had replaced Gimingham in 1881), and partly at the 230-year-old 
    Whitefriars glassworks of Harry Powell, an Oxford-educated chemist. The firm 
    of James Powell & Sons had been distinguished for its manufacture of coloured 
    (stained) glasses since 1851.15 Crookes also took photographs of the spectra 
    of molten glass while visiting the Nuttall bottle works in St Helens, the 
    owner being particularly keen to save his workforce's eyesight. Although 
    Nuttall had the opening in a glass furnace bricked up, leaving a small hole 
    for the spectroscope, the fiercely hot conditions cannot have been pleasant 
    for an 80-year-old man, and it was Gardiner who undertook much of the on-site 
    measurements. Several of the exposures taken lasted three-quarters of an 
    Crookes rapidly determined that any worker exposed to the brilliant light from 
    the furnace for three hours would receive a massive dose of infrared 
    radiation, and he recalled how the Palm House at Kew Gardens used pale-green 
    glass opaque to the infrared. He believed that ?spectacles or screens made of 
    this glass might be of use in glassworks if the workmen would use them?.16 By 
    1909, however, Crookes became aware of French and German research showing 
    that pathological changes to the eye lens were probably caused by ultraviolet 
    radiation and that French glassmakers had succeeded in developing a coloured 
    glass that dampened its transmission. This amber-coloured ?Euphos glass? 
    rapidly entered the catalogues of spectacle manufacturers all over the world, 
    the prescriptions being especially recommended for the goggles of sportsmen 
    and drivers of automobiles. Crookes and Gardiner believed that this kind of 
    tinted glass could be bettered if the absorption spectra of different types 
    of glass were accurately determined.
    It was not until the summer of 1911 that Crookes was able to report to the 
    Committee on successful formulations after appraising and comparing about 160 
    different combinations of metallic glass with white clear glass. In theory, 
    he told the Committee, ?it should be possible to make a glass which would be 
    opaque to the infrared and the ultraviolet, and my endeavours have been 
    directed for some time towards that end, but hitherto I have not been 
    successful?.17 Clearly, a compromise formulation would be necessary. 
    Interestingly, he first tested the suitability of each metal ion by cutting a 
    specimen of a pure metal into a thin plate 2 mm thick and using it as a 
    radiometer vane to determine the relative order in which heat was cut off. 
    Each metal was then tested spectroscopically to identify the limits of its 
    ultraviolet spectrum, as well as its opacity (percentage of light rays 
    transmitted) and colour in a tintometer. These tests had shown him that it 
    was worth experimenting with glass mixtures containing small quantities of 
    the oxides or salts of cerium, chromium, cobalt, copper, iron, lead, 
    manganese, neodymium, nickel, praesodymium and uranium. Meanwhile, the 
    government was badgering the Committee for results, but Crookes needed 
    another two years' intermittent research and the inspiration of Faraday's 
    glass researches before he was ready to go public.18
    An interim report on the Committee's work and conclusions was sent to the Home 
    Office in April 1911, after which the government gave a further ?100 towards 
    a fuller investigation of the effects of light absorption on vision because 
    it might have a bearing on the work of another government investigation of 
    the lighting in factories and workshops, in which Parsons was centrally 
    involved. The Committee decided, however, that it would need at least ?200 to 
    accomplish this. To pump the government for the additional ?100 the Committee 
    urged Crookes to write up his results quickly as proof that the research was 
    offering value for money. Crookes's private report was subsequently formally 
    presented to the Royal Society a month into his presidency in November 1913. 
    The Home Office was clearly impressed and presented the Committee with the 
    additional ?200 in January 1914. The paper announced a formulation that cut 
    off 90% of heat radiation, was opaque to ultraviolet light, and was 
    relatively free of colour so that objects remained clear to the eye. Crookes 
    noted that, besides glassworkers, there would be an advantage in preparing 
    such coloured or tinted glasses to prevent glare for people exposed to 
    sunlight reflected off cliffs, snow, or even electric light."During the 
    brilliant weather of the late summer [1911] I wore some of these spectacles 
    with great comfort; they took off the whole glare of the sun on chalk cliffs, 
    and did not appreciably alter the natural colours of objects. Lady Crookes, 
    whose eyes are more sensitive to glare or strong light than are my own, wore 
    them for several hours in the sun with great comfort.19"
    Crookes was not the first to prepare tinted lenses for leisure use, but here 
    was the origin of scientifically formulated Crookes lenses or modern 
    Crookes tested more than 300 tinted glasses for this swansong research 
    project, each formulation receiving a serial number that entered the glass 
    literature. The formulations he produced included ?specimens suitable for 
    spectacles adapted to all requirements?from Eyes of Youth to Eyes of Age? 
    (type 302). To cut off heat radiation he recommended Crookes Glass 246 (a 
    sage-green glass containing ferrous oxalate with red tartar and wood 
    charcoal) that eliminated 98% of the incident heat; for cutting out 
    ultraviolet light the best glass was Crookes 158, containing cerium borate 
    and ferric and chromic oxides; and for sunglasses the best choice was a pale 
    blue Crookes 249 (Chance's Crookes Glass A1) containing cerium nitrate with a 
    little ferric oxide and cobalt sulphate. All these glass formulations had 
    been prepared with the cooperation of Chance Brothers at Birmingham, who 
    exhibited samples at a Royal Society Conversazione in 1914,21 and by Powell's 
    Whitefriars glassworks in London.22 The five-year research project also 
    proved of considerable significance with the outbreak of war in August 1914. 
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