Page:Popular Science Monthly Volume 11.djvu/366

352 gradations until it becomes too feeble for distinct vision. Its aspects vary very much with the season of the year."

The crepusclescrepuscules [sic], or streaks of light from the sun, must not be mistaken for the zodiacal light; the former are sometimes visible between twilight and dark, the latter not until the shades of night have fully set in. Neither must the milky-way be mistaken for it by inexperienced observers, as the zodiacal light has a warm yellowish tinge unlike the cold white light of the milky-way.

Having endeavored to open the way for a consideration of the subject, we will now proceed to give a history of former observations.

There is no mention of any appearance of this light by very early writers. There is mention of Arcturus and the bands of Orion in the book of Job, and the constellations of the zodiac were assigned names at a very early date. The zodiacal belt was in use among the ancient Egyptians and Hindoos. If this light had been visible, it is highly probable some ancient writer would have spoken of it. Our author observes:

He is, however, of the opinion that it may have been overlooked or mistaken for the crepuscle at early dawn or twilight.

It has been supposed that Pliny, who wrote in the first century of the Christian era, alludes to it under the name of trabes or the δοκονς of the Greeks, but Humboldt thinks that Pliny refers to another matter.

Ammonius, in his life of Charlemagne, 807, mentions an appearance somewhat like the zodiacal light, but there was no reliable notice of it before it was described in Childrey's "Britannia Baconica" in 1661, which gives a brief description of its appearance and shape. The reference to it may be found in that work, at page 183.

It was reserved for Cassini to direct attention to its examination for the first time with earnest inquiry and interest. His first notice of it was on the evening of the 18th of March, 1683. He was watching in the west for other things, but was struck with the appearance of this luminous streak reaching far up in the sky. Like most discoverers, Cassini immediately formed a theory in regard to it after making but ten observations. This hypothesis, based on very insufficient data, has continued to the present time to influence the opinions of astronomers, and has retarded interest in observing its phenomena. Like most theories of the heavenly bodies in their first origin, it was doubtless erroneous, and, like the Ptolemaic theory of the solar system, it has materially interfered with the establishment of a correct knowledge of what is the truth.