STAR SPECTRA.
Without the spectroscope very little advance could have been made in the knowledge of stellar constitutions and motions. As long as sufficient light can be obtained to give a visible spectrum or affect a sensitive plate it does not matter in the slightest degree what the distance of the source is. There are one or two special devices that must be used in obtaining star spectra. The image of a star being a point, the slit method of observation cannot be used. Since the rays from the star are already parallel, the method of the objective prism may be applied. The prism is placed in front of the object glass, which is of long focus, and this gives an extremely narrow spectrum of the usual extent from red to violet. An appreciable width must be obtained before any accurate comparisons can be made. One method is to inter pose a cylindrical lens in the path of the light. A point source gives a line image with such a lens. This is a very con venient method for visual observation, and the cylindrical lens may be used for the eyepiece. Another way is to place the refracting edge of the objective prism parallel to the earth's equator. The long thin spectrum will be then at right angles to the apparent movement of the star. If,
now, the eliminating mechanism of the telescope be altered slightly in its rate, so as to go a little slow, there will be a trail produced on the negative by each point of colour, and thus the spectrum will be produced with its usual characteristics.
in diameter, on the collimator end. As the platform carries the slit across the image, the effect. of a- slit shutter is ob tained. Light from each part of the image will in turn pass through the slit, and, if a camera be placed behind it, an ordinary negative will be obtained. In stead of that, the light is led through a spectrometer arrangement as shown. The lays coming from the prism are reflectud by a mirror into the usual telescope tube, which is furnished with a slit s, instead of an eyepiece. The light is brought to a focus by the lens I, , forming a spectrum: at N, the place of the fixed plate-holder. N is separate from the platform. The ad There is, however, a difficulty in the objective prism method in keeping the terrestrial spectrum, added for com parison, in the same relative position with regard to the celestial one.