The wonders of modern mechanismA résumé of recent progress in mechanical, physical, and engineering science . ained by a prism is more refracted than the red end,and as a consequence the violet rays are relatively morespread out, so that the amount of space occupied by theserays is out of proportion to the relative difference of theirwave-length. This defect, which is termed the irration-ality of the prismatic spectrum, is quite absent from thatobtained by means of the diffraction grating, as the forma-tion of the spectrum has been seen in the latter case to besolely dependent upon the differe
The wonders of modern mechanismA résumé of recent progress in mechanical, physical, and engineering science . ained by a prism is more refracted than the red end,and as a consequence the violet rays are relatively morespread out, so that the amount of space occupied by theserays is out of proportion to the relative difference of theirwave-length. This defect, which is termed the irration-ality of the prismatic spectrum, is quite absent from thatobtained by means of the diffraction grating, as the forma-tion of the spectrum has been seen in the latter case to besolely dependent upon the different wave-lengths of therespective rays. If a Rowland concave grating is used no observingtelescope is required, a real image being formed that maybe thrown on a screen or photographic plate. 390 WONDERS OF MODERN MECHANISM. In other instruments the telescope itself can be movedin the horizontal plane, and thus a fine wire across theoccular can be brought into exact coincidence with anypart of the spectrum, and the position may be seen by theamount of motion of the telescope, which can be read off Fig. Principle of the Spectroscope.—a, prism; b, telescope through which the lightmay pass ; c, magnifying eye-piece; d, scale. from a suitable scale on the stand. For astronomical usethe spectroscope has a still different form, and is attachableto a telescope, through which the light of a star may bereceived. Owing to the way in which the rays are turned asidefrom their path in the prism, the telescope and collimatorin the ordinary forms of spectroscope must be placed at anangle to each other, but by an ingenious arrangement direct-vision spectroscopes are made that have the two tubes inline, so that they can be conveniently handled, or evencarried in the pocket. This is possible because crown andflint glass differ in their power of refraction and dispersion. THE SPECTROSCOPE. 391 The crown glass, being less dispersive, is used to bendback the slightly refracted and much-dispe
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