Halley's Comet
Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict its 1759 return. Streams of vapour observed during the comet's 1835 apparition prompted astronomer Friedrich Wilhelm Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit.
The 1910 approach, which came into naked-eye view around 10 April and came to perihelion on 20 April, was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained. Furthermore, the comet made a relatively close approach of 0.15 AU, making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet. One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen, which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet." His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public. In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.
Biela's Comet
The comet appeared as predicted during its 1832 apparition, when it was first recovered by John Herschel on 24 September. The orbital elements and ephemeris calculated by Olbers for this return created something of a popular sensation, as they showed that the comet's coma would likely pass through the Earth's orbit during a close approach on October 29. Subsequent predictions, in the media of the time, of the Earth's likely destruction overlooked the fact that the Earth itself would not reach this point until November 30, a month later, as pointed out by François Arago in an article designed to allay public fears. Despite this, the fact that Biela's Comet was the only comet known to intersect the Earth's orbit was to make it of particular interest, both to astronomers and the public, during the 19th century.