A Handbook of Descriptive and Practical Astronomy; Instruments and Practical Astronomy (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1890 Excerpt: ...given by various points of its surface or its surroundings, and these have proved of the highest interest and importance. For this department of work a fairly large image of the Sun is needed, and therefore a telescope of somewhat long focus is required to throw an image of the Sun upon the slit. When simply the general spectrum of the Sun's surface is to be examined, it is often sufficient to point the spectroscope towards the Sun without attaching it to any telescope at all, or it may even be turned towards a bright cloud or the sky. So used, the spectroscope is often spoken of as an integrating spectroscope; but when a definite image of the source of light is formed upon the slit, so that the special features of the spectrum of any particular portion of the light can be distinguished, it is said to be an analyzing spectroscope; the spectroscope remaining the same in the two cases, but the mode of its employment being altered. The first details of the solar surface to be separately examined were the spots. The general appearance presented by the spectra of the different parts of a spot is shown in Fig. 151. The left-hand figure shows the position of the slit on the spot. The centre one shows the portion of the spot covered by the slit, which is what an observer looking directly at the Sun through the slit would see. The portions AS, EF, at either end of the slit, belong to the clear surface of the Sun, the middle part CD to the umbra of the spot, and the parts between BC and DE to the penumbra. Corresponding to these 5 parts we have 5 narrow spectra, the centre one that of the umbra, on each side of it a spectrum of the penumbra, and outside these an ordinary solar spectrum. In spite of their darkness as compared with the general surface spots give a perf...