Nagel Anomaloscope Model 1 (c. 1907)

The Nagel Anomaloscope Model 1 is considered the "gold standard" clinical instrument for the precise diagnosis and classification of red-green color vision deficiencies. Invented by the German physiologist and ophthalmologist Willibald A. Nagel in 1907, the device was developed to provide a more rigorous scientific evaluation than the simple color-matching cards or wool skeins common in the late 19th century. It operates on the principle of the Rayleigh Match, named after Lord Rayleigh, which requires a subject to mix specific wavelengths of red and green light to match a standard monochromatic yellow light.

Technically, the Model 1 is a sophisticated spectroscope consisting of a viewing ocular, a compound direct-vision prism, and three precisely calibrated entrance slits that define its primary colors: red (⁵$666$ nm), green (⁶$546$ nm), and yellow (⁷$589$ nm).⁸ The instrument presents the user with a circular field split into two halves:

The Bottom Half: A spectral yellow light whose brightness can be adjusted.

The Top Half: A mixture of red and green light where the proportions are variable.

By analyzing the specific ratio of red to green a person requires to perceive a perfect match, clinicians can distinguish between normal trichromats, anomalous trichromats (who need more red or green than average), and dichromats (who can match the yellow to any red-green mixture by simply adjusting brightness). Although Schmidt & Haensch manufactured the Model 1 until 1983, it remains the definitive tool for characterizing the many phenotypic variations in X-linked color vision disorders. Modern research has even used the instrument to debunk "seasonal variations" in human color perception, proving instead that the Nagel’s internal prisms are highly sensitive to ambient temperature and voltage fluctuations.

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