148. _The Dispersion Spectroscope._--The essential parts of the _dispersion_ spectroscope are shown in Fig. 163. These are the _collimator tube_, the _prism_, and the _telescope_. The collimator tube has a narrow slit at one end, through which the light to be examined is admitted, and somewhere within the tube a lens for condensing the light. The light is dispersed on passing through the prism: it then passes through the objective of the telescope, and forms within the tube an image of the spectrum, which is examined by means of the eye-piece. The power of the spectroscope is increased by increasing the number of prisms, which are arranged so that the light shall pass through one after another in succession. Such an arrangement of prisms is shown in Fig. 164. One end of the collimator tube is seen at the left, and one end of the telescope at the right. Sometimes the prisms are made long, and the light is sent twice through the same train of prisms, once through the lower, and once through the upper, half of the prisms. This is accomplished by placing a rectangular prism against the last prism of the train, as shown in Fig. 165. [Illustration: Fig. 164.] [Illustration: Fig. 165.] 149. _The Micrometer Scale._--Various devices are employed to obtain an image of a micrometer scale in the tube of the telescope beside that of the spectrum. [Illustration: Fig. 166.] One of the simplest of these methods is shown in Fig. 166. _A_ is the telescope, _B_ the collimator, and _C_ the micrometer tube. The opening at the outer end of _C_ contains a piece of glass which has a micrometer scale marked upon it. The light from the candle shines through this glass, falls upon the surface of the prism _P_, and is thence reflected into the telescope, where it forms an enlarged image of the micrometer scale alongside the image of the spectrum. [Illustration: Fig. 167.] 150. _The Comparison of Spectra._--In order to compare two spectra, it is desirable to be able to see them side by side in the telescope. The images of two spectra may be obtained side by side in the telescope tube by the use of a little rectangular prism, which covers one-half of the slit of the collimator tube, as shown in Fig. 167. The light from one source is admitted directly through the uncovered half of the slit, while the light from the other source is sent through the covered portion of the slit by reflection from the surface of the rectangular prism. This arrangement and its action will be readily understood from Fig. 167. [Illustration: Fig. 168.] 151. _Direct-Vision Spectroscope._--A beam of light may be dispersed, without any ultimate deflection from its course, by combining prisms of crown and flint glass with equal refractive, but unequal dispersive powers. Such a combination of prisms is called a _direct-vision_ combination. One of three prisms is shown in Fig. 168, and one of five prisms in Fig. 169. [Illustration: Fig. 169.] [Illustration: Fig. 170.]