Blue
2 of the 7 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 2026 Editor's Note: Blue turns out to be the rarest color in nature: almost no animal makes a blue pigment, and the blue of a jay's feather or a morpho's wing is a trick of microscopic structure scattering light. True blue pigments have always been costly — ultramarine, ground from lapis lazuli, once exceeded gold — and new ones are so rare that the accidental discovery of YInMn blue in a lab in 2009 was the first new inorganic blue in two centuries. (Ed: BR 2026-06-21)
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.