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Perhaps a longer account from the RS Crookes report might be worthwhile for interest's sake:-


Douglas Denny.
Chichester. England.
===========


Quote:-

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 
hour.

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 
sunglasses.20

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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