60 



CHEMICAL PHYSICS. 



employed, of which Fig. 21 represents the single-prism spectroscope, 

 which is most largely used. It consists of the prism P and of 

 three telescopes directed toward it. The light is permitted to enter 

 the tube A through a narrow slit at its distal end, while at the 

 other end a convex lens, called the collimator, serves to collect the 

 light into nearly parallel beams. These beams pass through the 

 prism where the dispersion is brought about, and the spectrum thus 

 formed is observed at d through the telescope B. Through the third 



FIG. 21. 



Spectroscope. 



tube, C, a fine scale for measurement of the relative position of lines 

 or colors is reflected to the eye of the observer. This scale is usually 

 photographed on glass, and when illuminated by a candle or some 

 other stronger light the image of the scale is directed upon the face 

 of the prism in such a manner that it is reflected down the axis of 

 the observing telescope, and is seen above or below the spectrum, 

 according to the direction given to the axis of the scale tube. The 

 glass vessel a serves as a container for a liquid to be examined spec- 

 troscopically. 



A second form of spectroscope, known as the direct-vision spec- 

 troscope, is shown in Fig. 22. It consists of a cylindrical tube 

 provided with ocular and containing from three to seven prisms, 



