A History of Light & Colour Measurement: Science in the by Sean F. Johnston

By Sean F. Johnston

This can be a heritage of the hidden workings of actual technology - a technical endeavour embedded in a social context. It argues that this 'undisciplined' topic, straddling academia, trade and law, will be ordinary not just of twentieth century technology, yet of its destiny.

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But a variety of processes-social, technological and scientific-transformed the brightness of light in the late 19th century from the passing concern of a few disparate individuals to a subject employed and studied by groups. It was shaped by new perceptions and expectations, too. This cultural transformation was accompanied by the growing identification of the subject as a part of physical science, steering it towards an increasingly quantitative expression. Despite this recategorization, photometry remained, by the end of the century, an undisciplined and fragmented study.

In 1851, though, W R Dawes criticized what he saw as weaknesses of previous estimates: The differences among observers of great experience and celebrity are much greater than would probably be imagined by those who have not been led to examine the subject, and clearly show that widely different scales of magnitude have been adopted. . According to Campbell's classification, stellar magnitudes at this time were of the first class, merely ranking values along an unreliable scale. 5, discrepancies corresponding to differences of about eightfold in estimated i ~ i t e ~ i s i t ~ ~ .

This was a device in the form of a telescope incorporating an adjustable aperture wheel and graticule with scribed letters. The appropriate aperture, calibrated in terms of intensity, was selected to make the smaller letters illegible while the telescope was pointed at the light source of interest. 30 On Herschel's novel astronomical style, see Schafer S 1981 'Uranus and the establishment of Herschel's astronomy' J. Hist. Astron. 12 11-26. 31 Michell J 1767 'An inquiry into the probable parallax, and magnitude of the fixed stars, from the quantity of light which they afford us, and the particular circumstances of their situation' Phil.

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