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    Re: Basics of computing sunrise/sunset
    From: Douglas Denny
    Date: 2009 Jun 18, 11:14 -0700

    Hello Brad,
    
    I would not like to be quoted as being definite about anything on this matter 
    but the best suggestion is that anything before 1910 might be suspect.  Sir 
    William Crookes was asked to look into the matter in 1906 and the first 
    Crookes Glass formulations came into being.
    
    The report by the Royal Society can be found at:-
    
    http://rsnr.royalsocietypublishing.org/content/61/3/301.full
    
    The early lab instuments using neutral density filters used literally 'smoked' 
    glass:  glass placed in a smokey gas flame where carbon particles adhere.  I 
    am not sure about other "coloured" glass of that era for sextants but the 
    only way to be sure is to have spectroscopic analysis done - not easy unless 
    you have friendly university labs available.
    
    Regards,
    
    Douglas Denny.
    
    Chichester.  England.
    =======================
    
    An extract of the R.S. report:-
    
    The Royal Society's Glass Workers' Cataract Committee; Sir William Crookes and 
    the development of sunglasses
    
    Abstract
    
    After the inclusion of a number of industrial diseases and injuries in the 
    Workmen's Compensation Acts of 1896 and 1906, the government asked the Royal 
    Society to investigate how and why glare and heat apparently caused 
    glassworkers to develop cataracts during their working lives. The activities 
    between 1908 and 1928 of the Glass Workers' Cataract Committee, which was 
    made up of chemists, physiologists and ophthalmologists, are discussed. 
    Emphasis is placed on the attempts by the octogenarian William Crookes (PRS 
    1913?15) to formulate a spectacle glass that was opaque to infrared and 
    ultraviolet radiation. While providing relief for industrial workers, the 
    research also laid the foundation for the modern sunglasses industry. Other 
    significant work of the Committee concerned the biochemistry of the eye.
    
    
    
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