A _direct-vision spectroscope_ (Fig. 170) is one in which a direct-vision combination of prisms is employed. _C_ is the collimator tube, _P_ the train of prisms, _F_ the telescope, and _r_ the comparison prism. [Illustration: Fig. 171.] 152. _The Telespectroscope._--The spectroscope, when used for astronomical work, is usually combined with a telescope. The compound instrument is called a _telespectroscope_. The spectroscope is mounted at the end of the telescope in such a way that the image formed by the object-glass of the telescope falls upon the slit at the end of the collimator tube. A telespectroscope of small dispersive power is shown in Fig. 171; _a_ being the object-glass of the telescope, _cc_ the tube of the telescope, and _e_ the comparison prism at the end of the collimator tube. A more powerful instrument is shown in Fig. 172. _A_ is the telescope, _C_ the collimator tube of the spectroscope, _P_ the train of prisms, and _E_ the telescope tube. Fig. 173 shows a still more powerful spectroscope attached to the great Newall refractor (18). [Illustration: Fig. 172.] [Illustration: Fig. 173.] 153. _The Diffraction Spectroscope._--A _diffraction_ spectroscope is one in which the spectrum is produced by reflection of the light from a finely ruled surface, or _grating_, as it is called, instead of by dispersion in passing through a prism. The essential parts of this instrument are shown in Fig 174. This spectroscope may be attached to the telescope in the same manner as the dispersion spectroscope. When the spectroscope is thus used, the eye-piece of the telescope is removed. [Illustration: Fig. 174.] Spectra. 154. _Continuous Spectra._--Light from an incandescent solid or liquid which has suffered no absorption in the medium which it has traversed gives a spectrum consisting of a continuous colored band, in which the colors, from the red to the violet, pass gradually and imperceptibly into one another. The spectrum is entirely free from either light or dark lines, and is called a _continuous spectrum_. 155. _Bright-Lined Spectra._--Light from a luminous gas or vapor gives a spectrum composed of bright lines separated by dark spaces, and known as a _bright-lined spectrum_. It has been found that the lines in the spectrum of a substance in the state of a gas or vapor are the most characteristic thing about the substance, since no two vapors give exactly the same lines: hence, when we have once become acquainted with the bright-lined spectrum of any substance, we can ever after recognize that substance by the spectrum of its luminous vapor. Even when several substances are mixed, they may all be recognized by the bright-lined spectrum of the mixture, since the lines of all the substances will be present in the spectrum of the mixture. This method of identifying substances by their spectra is called _spectrum analysis_.