Of late simpler constructions have come into use of which an excellent type is the spectroscope designed by Mr. Evershed and shown in diagram in Fig. 144. Here the path of the rays is from the slit through the collimator objective, then through a very powerful direct vision system, giving a dispersion of 30° through the visible spectrum, then by reflection from the mirror through a second such system, and thence to the observing telescope. The mirror is rotated to get various parts of the spectrum into view, and the micrometer screw that turns it gives means for making accurate measurement of wave lengths. There are but five reflecting surfaces in the prism system (for the cemented prism surfaces do not count for much) as against more than 20 in one of the older instruments of similar power, and as in other direct vision systems the spectrum lines are substantially straight instead of being strongly curved as with multiple single prisms. The result is the light, compact, and powerful spectroscope shown complete in Fig. 145, equally well fitted for observing the sun’s prominences and the detailed spectrum from his surface. [Illustration: FIG. 145.—Evershed Solar Spectroscope.] In most of the solar spectroscopes made at the present time the prisms are replaced by a diffraction grating. The original gratings made by Fraunhofer were made of wire. Two parallel screws of extremely fine thread formed two opposite sides of a brass frame. A very fine wire was then wound over these screws, made fast by solder on one side of each, and then cut away on the other, so as to leave a grating of parallel wires with clear spaces between. Today the grating is generally ruled by an automatic ruling engine upon a polished plate of speculum metal. The diamond point carried by the engine cuts very smooth and fine parallel furrows, commonly from 10,000 to 20,000 to the inch. The spaces between the furrows reflect brilliantly and produce diffraction spectra.[22] [22] For the principle of diffraction spectra see Baly, Spectroscopy; Kayser, Handbuch d. Specktroskoie or any of the larger textbooks of physics. When a grating is used instead of prisms the instrument is commonly set up as shown in Fig. 146. Here _A_ is the collimator with slit upon which the solar image light falls, _B_ is the observing telescope, and _C_ the grating set in a rotatable mount with a fine threaded tangent screw to bring any line accurately upon the cross wires of the ocular. [Illustration: FIG. 146.—Diagram of Grating Spectroscope.] The grating gives a series of spectra on each side of the slit, violet ends toward the slit, and with deviations proportional to 1, 2, 3, 4, etc., times the wave length of the line considered. The spectra therefore overlap, the ultra violet of the second order being superimposed on the extreme red of the first order and so on. Colored screens over the slit or ocular are used to get the overlying spectra out of the way.