Blue
2 of the 6 encyclopedias on this shelf carry an entry for Blue. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
the color of the clear sky. The student of physics would say that it is the color produced by light having waves of a certain length. In the rainbow blue lies between green and indigo. Blue dyes are obtained from several sources. The Alexandria blue used by the ancient Egyptians was made from copper and lime. Cobalt blue is a dye of particular beauty. Prussian blue is a compound of iron. Saxony blue is prepared from indigo. Ultramarine blue is obtained from the mineral, lapis lazuli . Blue is a favorite with the dyer. Commercial blues are obtained chiefly from indigo and aniline preparations. Blue vitriol is sulphate of copper. Pottery blues are obtained from cobalt. The sapphire and the turquoise are blue gems. The Covenanters of Scotland carried a blue banner. They were called true blue Presbyterians. A bluestocking is a woman noted for intellectuality, possibly to the exclusion of womanly graces. Navy blue is the color of the British naval uniform. Blue is also the color of the American uniform; hence, our soldiers are called, not infrequently, "boys in blue." See Color
Collier's New Encyclopedia (1921)
one of the seven colors into which the rays of light divide themselves when refracted through a glass prism, seen in nature in the clear expanse of the heavens; also a dye or pigment of this hue. The blue pigments in common use by artists are few in number, and consist of native and artificial ultramarine, cobalt, indigo, and Prussian blue. Genuine ultramarine, prepared from the mineral lapis lazuli, and ordinary cobalt blue, sold for artists' work, are permanent colors. Prussian blue and indigo are highly useful colors, since it is only these that yield dark blues, and only from them, mixed with yellows or browns, s, that strong greens can be obtained.
What Has Changed in the Last Century
When Aiton's wrote in 1910 that "commercial blues are obtained chiefly from indigo and aniline preparations," the shift it describes was already nearly complete: BASF began selling synthetic indigo in 1897, and within two decades it had all but wiped out the natural indigo plantations of India. The artist's short list Collier's gave in 1921 — ultramarine, cobalt, indigo, Prussian blue — has since gained the copper phthalocyanine blues, patented in the 1930s and now the most-used blue pigment in the world, in everything from printing ink to denim-blue plastics. Two other things have moved: the sky's blue is now routinely explained to schoolchildren as Rayleigh scattering, and the blue of a jay's feather is known to be no pigment at all but light scattered by microscopic structure. Aiton's aside that a "bluestocking" is a woman intellectual "possibly to the exclusion of womanly graces" is the sentence a modern reader will notice first; the word survives only as a period curiosity.
Written for this edition, 2026.