One of the mysteries of the English language finally explained.
1Medicine the ratio of the quantity of haemoglobin to the number of red blood cells present in a blood sample (now rare).
2Astronomy a number representative of the colour (and hence the temperature) of a star, equal to the difference in its magnitude (brightness) at two chosen wavelengths, typically blue and green-yellow, or ultraviolet and blue.
3Geology a number expressing the percentage of dark-coloured minerals (typically ferromagnesian ones) in an igneous rock.
Mid 19th century; earliest use found in British Journal Homoeopathy.
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