ATTACHMENT OF SPECTROSCOPE. TO TELESCOPE.
The method of adapting the spectro scope to the end of the large telescope for solar observation is shown by Fig. 805. The collimator is attached to a sliding base similar in construction to the metal slide rest used with lathes. The base is capable of motion in two directions at right angles, and is adjusted by means of two fine micrometer screws, one of which is seen at the left hand of the movable base. The amount of movement of the slit is registered by a circular scale round the screw head. When the telescope T of the spectroscope is fixed in position on a certain line in the spectrum, the rotation of these screws will bring the slit, to different parts of the real image, and will give information as to the occurrence of the element showing that line at any part of the object. if the grating or prism be fixed, the line must stay in position on the cross-wires. III the instrument
shown, the spectrum is caused by a reflec tion grating. To avoid unsteadiness, the telescope of the spectroscope is clamped rigidly in one position, and the grating is rotated to bring different parts of the spectrum into the field of observation. The telescope is an equatorial, and the mechanism for eliminating the effect of the earth's motion is partly shown. With a long train of prisms, as in Mill's spectrograph, where the exposure must necessarily he protracted, the prisms must be protected from changes of tem perature, or the deviation will be altered. The camera requires the same sliding base arrangement for the tube carrying its lens, when enlarging the real image. When no enlargement is wanted, the slid ing base carries the dark-slide. Such an arrangement needs careful treatment.