Page:Popular Science Monthly Volume 42.djvu/561

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if large bodies are dealt with, the cooling must take a very long time.

The latest view put forward is, that these bodies are produced by the sudden meeting in space of two swarms or streams of meteoritic matter, each traveling with a considerable velocity, the sudden bright light being due to the collisions of the particles composing the swarms; and this hypothesis explains very well not only the sudden outburst, but the rapid decrease in brightness, due to the fact that only small particles are dealt with, and these must cool and dim quickly.

The appearance of the present new star, or "Nova," in the constellation of Auriga, was first announced by an anonymous post-card received at the Royal Observatory, Edinburgh. Why the post-card was sent anonymously remains a mystery; but the extraordinary reticence of the writer does not make any difference to the immortality of the discoverer; for while, on the one hand, newly discovered comets, which are also of an apparently temporary nature, are always associated with the names of those who first observe them, new stars, on the other hand, are always referred to by the name of the constellation in which they appear.

The instrument now used to obtain observations of these strange visitors consists of a combination of an object-glass; a prism, which is placed outside the object-glass, and a camera. The function of the prism is to separate the million strands of colored light which go to make white light; that of the object-glass is to collect each color, concentrating it at the same time, so that finally we get a fine line of rainbow color.

This method of obtaining a spectrum is by no means modern, but was suggested and used by the German optician Fraunhofer about the year 1814. He placed a prism before the object-glass of a theodolite, and in this way was the first to observe the spectra of some of the stars. By the use of this method, whether the eye or the photographic plate is used, the so-called "spectrum" of the body under observation can be studied without any difficulty. The length of the exposures required when photography is employed for stars of different magnitude varies very considerably; for the brightest a few minutes are generally ample, but for those of much smaller magnitude a space of two or three hours is by no means too long.

The spectra that are thus obtained are of various kinds, as various classes of so-called stars are observed. Some consist of bright lines on a dark background, others of dark lines on a bright background while a mixture of both these is met with. These variations in spectra depend upon the fact that any substance that is heated sufficiently to emit light whether